fix(content-router): token-measure lossless folds at the acceptance gate (#1772)

## Description

Unit-mismatch bug in the compression acceptance gate. `router.apply()`
computes `compression_ratio` from `len(text.split())` (word count), but
a **lossless** search/log fold (`compact_lossless`) saves **bytes** by
collapsing a repeated path prefix into a single heading — word count
stays flat or even *rises* (the heading adds a word). So the gate saw
`ratio ≥ 1.0` and discarded every free, byte-recoverable win as
`ratio_too_high`. (Raising the floor to 1.0 in #1771 did **not** fix
this — the word-ratio was already ≥ 1.0.)

Measure lossless results (those whose `strategy_chain` carries a
`lossless_*` entry) by **byte ratio** at the gate and in the result
cache — the real saving. Lossy strategies are unchanged (word count
tracks their token savings), and the reversibility gate is untouched
(`LOG`/`SEARCH`/`DIFF` aren't in `LOSSY_UNMARKED_STRATEGIES`). The
excluded-tool and bash-search paths already bypass this gate via
`continue`; this fixes the **main strategy dispatch** (the lossless-mode
`LOG`/`SEARCH`/`DIFF` path).

Follow-up to #1771.

Closes #

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- At the `apply()` acceptance gate: compute `accept_ratio` = byte ratio
for lossless results (`strategy_chain` has `lossless_*`), else the
existing word ratio. Gate + result-cache entry now use `accept_ratio`.
- Added an end-to-end regression test that drives the full
`router.apply()` path.

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check`)
- [x] Type checking passes (`mypy headroom`)
- [x] New tests added for new functionality
- [ ] Manual testing performed

### Test Output

```text
tests/test_lossless_mode.py::test_router_apply_accepts_lossless_search_byte_measured PASSED
tests/test_content_router_tool_role_reversibility.py .......... (10 passed)
# broader (pre-move) sweep on the same change:
tests/test_lossless_mode.py / test_transforms/test_content_router.py /
test_lossless_excluded_compaction.py / test_bash_search_lossless_fold.py — 121 passed
ruff check headroom/transforms/content_router.py  -> All checks passed!
mypy headroom/transforms/content_router.py         -> Success: no issues found
```

## Real Behavior Proof

- Environment: local worktree, Python 3.12, `PYTHONPATH` pinned to the
branch.
- Exact command / steps: new regression test constructs a single-file
grep result, runs it through `ContentRouter(lossless=True).apply(...)`,
and asserts the tool output is byte-smaller and recovers exactly
(`search_unheading(out) == original`).
- Observed result: before this fix the fold was rejected (`out ==
original`, counted `ratio_too_high`); after, it's applied (`len(out) <
len(original)`, marker-free, byte-exact recovery). The test also asserts
the fold's word count is ≥ the original's, so the test is meaningless if
"fixed" by word count.
- Not tested: no live end-to-end proxy run; validated via the full
`apply()` path in unit tests.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective
- [x] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable (handled at release
time)

## Additional Notes

Why prior tests missed it: `compress()` and `_apply_strategy_to_content`
return the folded result directly and never touch the `apply()`
acceptance gate, so the existing lossless-mode unit tests (which call
those) passed while the real proxy path silently discarded the fold. The
new test exercises `apply()` end-to-end.

headroom/transforms/content_router.py

@@ -3410,7 +3410,26 @@ class ContentRouter(Transform):
                     compressor_timing.get(strategy_key, 0.0) + compress_ms
                 )
 
-                if result.compression_ratio < min_ratio:
+                # Lossless folds (search/log/diff via compact_lossless) shrink by
+                # collapsing repeated path prefixes, but the gate's default ratio
+                # is word count — which barely moves (a heading line can push it
+                # >1.0), discarding a free, recoverable win. Measure lossless
+                # results by REAL TOKEN count (what actually costs money/context),
+                # not words and not bytes: accept iff tokens genuinely drop. The
+                # excluded/bash paths already bypass this gate; this fixes the
+                # main strategy dispatch.
+                is_lossless = any(
+                    s.startswith("lossless_")
+                    for s in (getattr(result, "strategy_chain", None) or [])
+                )
+                if is_lossless and getattr(result, "original", None):
+                    orig_tok = tokenizer.count_text(result.original)
+                    accept_ratio = (
+                        tokenizer.count_text(result.compressed) / orig_tok if orig_tok else 1.0
+                    )
+                else:
+                    accept_ratio = result.compression_ratio
+                if accept_ratio < min_ratio:
                     # tool ground truth must stay reversible — a lossy summarizer
                     # (kompress/text/code) that emitted no CCR retrieve marker is
                     # unrecoverable, so the agent would act on a fabricated summary
@@ -3433,7 +3452,7 @@ class ContentRouter(Transform):
                     self._cache.put(
                         content_key,
                         result.compressed,
-                        result.compression_ratio,
+                        accept_ratio,
                         result.strategy_used.value,
                     )
                     if netcost_enabled and not self._net_cost_allows(
@@ -3450,11 +3469,9 @@ class ContentRouter(Transform):
                         continue
                     result_slots[slot_idx] = {**message, "content": result.compressed}
                     transforms_applied.append(
-                        f"router:{result.strategy_used.value}:{result.compression_ratio:.2f}"
-                    )
-                    compressed_details.append(
-                        f"{result.strategy_used.value}:{result.compression_ratio:.2f}"
+                        f"router:{result.strategy_used.value}:{accept_ratio:.2f}"
                     )
+                    compressed_details.append(f"{result.strategy_used.value}:{accept_ratio:.2f}")
                     if slot_idx in frozen_unlock_slots:
                         transforms_applied.append("router:netcost_frozen_unlock")
                         route_counts.setdefault("netcost_frozen_unlocked", 0)

tests/test_lossless_mode.py

@@ -279,6 +279,42 @@ def test_router_lossless_never_emits_marker_various_inputs() -> None:
         _assert_no_marker(router.compress(s, context="").compressed)
 
 
+def test_router_apply_accepts_lossless_search_token_measured() -> None:
+    """Regression: the acceptance gate in router.apply() measured WORD count, so
+    a lossless search fold — which cuts TOKENS by collapsing a repeated path
+    prefix while word count stays flat or *rises* (the heading adds a word) — was
+    wrongly discarded as ratio_too_high. The gate now measures lossless results
+    by real token count, so the free, recoverable win is applied. (compress()/
+    _apply_strategy_to_content bypass this gate, which is why unit tests above
+    never caught it — the bug only appears through the full apply() path.)
+    """
+    from headroom.providers import OpenAIProvider
+    from headroom.tokenizer import Tokenizer
+    from headroom.transforms.lossless_compaction import search_heading
+
+    tok = Tokenizer(OpenAIProvider().get_token_counter("gpt-4o"), "gpt-4o")
+    router = ContentRouter(ContentRouterConfig(lossless=True))
+    grep = "".join(
+        f"headroom/transforms/content_router.py:{i}:    identifier_{i} = compute(value)\n"
+        for i in range(1, 60)
+    )
+    # The fold does NOT reduce word count (the heading even adds one) — this is
+    # exactly what made the old word-count gate reject it.
+    assert len(search_heading(grep).split()) >= len(grep.split())
+
+    messages = [
+        {
+            "role": "assistant",
+            "tool_calls": [{"id": "c1", "function": {"name": "find_refs", "arguments": "{}"}}],
+        },
+        {"role": "tool", "tool_call_id": "c1", "content": grep},
+    ]
+    out = router.apply(messages, tok).messages[1]["content"]
+    assert tok.count_text(out) < tok.count_text(grep)  # accepted: fewer TOKENS
+    assert search_unheading(out) == grep  # byte-exact recovery
+    _assert_no_marker(out)
+
+
 # --------------------------------------------------------------------------
 # Token-delta measurement (informational)
 # --------------------------------------------------------------------------