fix(compression): report source-line span in CCR compression marker (#2597)

## Description

The compression marker read `[N items compressed to M. Retrieve more:
hash=...]`, where `items` counts whitespace-split **words**, not lines.
So five lines of tool output could show as `[122 items compressed to
27...]`. A reader can't map "items" to lines and can't tell "this line
was compressed away" from "this line was never in the output" — absence
reads as evidence of absence, which per the report led to a materially
wrong conclusion.

Closes #2586

## Type of Change

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

## Changes Made

- Annotate the marker with the source line count — `[N items compressed
to M (from L source lines). Retrieve more: hash=...]` — at both marker
sites: `KompressCompressor.compress` / `compress_batch`
(`kompress_compressor.py`) and the remote path (`kompress_remote.py`).
- The machine-parsed `Retrieve more: hash=` token is left byte-for-byte
unchanged, so CCR detection/retrieval is unaffected.

Scope note: I intentionally kept the existing `items compressed to`
phrasing rather than reword the unit, to avoid churning the marker
format that's referenced across ~12 test fixtures and the `config.py`
template. This is the minimal honesty fix; happy to go further (e.g.
line-unit counts or unifying with the `config.py` template) if you'd
prefer — see the issue thread where I asked about wording.

## Testing

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

### Test Output

```text
$ uv run pytest tests/test_compression_units.py tests/test_compression_batches.py \
    tests/test_ccr_marker_policy.py tests/test_ccr_tool_injection.py tests/test_session_probes.py -q
92 passed

$ uv run pytest tests/test_ccr_marker_policy.py -q
8 passed   # incl. new test_source_line_span_marker_is_still_detected

$ uv run ruff check headroom/transforms/kompress_compressor.py headroom/transforms/kompress_remote.py tests/test_ccr_marker_policy.py
All checks passed!
```

## Real Behavior Proof

- Environment: headroom @ main, Python 3.14, uv
- Exact command / steps: added a marker in the new enriched format and
ran it through the CCR marker detector.
- Observed result: the retrieval hash is still detected from `[122 items
compressed to 27 (from 5 source lines). Retrieve more: hash=...]`;
existing compression/CCR suites unchanged.
- Not tested: mypy not run locally; the full model-backed compress()
marker path isn't unit-exercised (needs a real backend), so the new test
targets the parser boundary instead.

## 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
- [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
- [x] I did **not** edit `CHANGELOG.md`

## Additional Notes

Wording is adjustable per the issue discussion. The `config.py` marker
template (a different code path with `Omitted`/`Expires` fields) is left
untouched to keep this focused on the Kompress marker the report hit.

headroom/transforms/kompress_compressor.py

@@ -1452,8 +1452,14 @@ class KompressCompressor(Transform):
                 cache_key = self._store_in_ccr(ccr_source, compressed, ccr_source_tokens)
                 if cache_key:
                     result.cache_key = cache_key
+                    # Report the source line span so a reader can tell content was
+                    # compressed away rather than absent — "items" counts words, which
+                    # does not map to lines and reads as evidence of absence (#2586).
+                    source_lines = ccr_source.count("\n") + 1
+                    line_word = "line" if source_lines == 1 else "lines"
                     result.compressed += (
-                        f"\n[{n_words} items compressed to {compressed_count}."
+                        f"\n[{n_words} items compressed to {compressed_count}"
+                        f" (from {source_lines} source {line_word})."
                         f" Retrieve more: hash={cache_key}]"
                     )
 
@@ -1809,8 +1815,14 @@ class KompressCompressor(Transform):
                 cache_key = self._store_in_ccr(ccr_source, compressed, ccr_source_tokens)
                 if cache_key:
                     result.cache_key = cache_key
+                    # Report the source line span so a reader can tell content was
+                    # compressed away rather than absent — "items" counts words, which
+                    # does not map to lines and reads as evidence of absence (#2586).
+                    source_lines = ccr_source.count("\n") + 1
+                    line_word = "line" if source_lines == 1 else "lines"
                     result.compressed += (
-                        f"\n[{n_words} items compressed to {compressed_count}."
+                        f"\n[{n_words} items compressed to {compressed_count}"
+                        f" (from {source_lines} source {line_word})."
                         f" Retrieve more: hash={cache_key}]"
                     )
 

headroom/transforms/kompress_remote.py

@@ -129,9 +129,14 @@ class RemoteKompressCompressor:
             cache_key = store_kompress_in_ccr(content, compressed, result.original_tokens)
             if cache_key:
                 result.cache_key = cache_key
+                # Report the source line span so a reader can tell content was
+                # compressed away rather than absent (#2586).
+                source_lines = content.count("\n") + 1
+                line_word = "line" if source_lines == 1 else "lines"
                 result.compressed += (
                     f"\n[{result.original_tokens} items compressed to "
-                    f"{result.compressed_tokens}. Retrieve more: hash={cache_key}]"
+                    f"{result.compressed_tokens} (from {source_lines} source {line_word})."
+                    f" Retrieve more: hash={cache_key}]"
                 )
 
         return result

tests/test_ccr_marker_policy.py

@@ -27,6 +27,13 @@ def test_has_new_ccr_markers_filters_replayed_forwarded_markers() -> None:
     )
 
 
+def test_source_line_span_marker_is_still_detected() -> None:
+    # The compressor annotates the count with a source-line span (#2586); the
+    # retrieval hash must still be extracted from the enriched marker.
+    marker = "[122 items compressed to 27 (from 5 source lines). Retrieve more: hash=c00eb437e5e5c00eb437e5e5]"
+    assert _hashes(marker) == ["c00eb437e5e5c00eb437e5e5"]
+
+
 def test_has_new_ccr_markers_detects_hash_not_seen_in_previous_forward() -> None:
     old = "[100 items compressed to 10. Retrieve more: hash=abc123def456abc123def456]"
     new = "[50 items compressed to 5. Retrieve more: hash=deadbeefdeadbeefdeadbeef]"