fix(dashboard): deduplicate repeated savings metrics (#1804)

## Description

The session dashboard repeats the same savings and performance numbers
in adjacent places. `proxy_compression_saved` appears in several
captions and detail rows, and average overhead and TTFB appear both in
the hero area and again in Performance without adding new context.

This narrows the non-hero dashboard presentation so repeated session
metrics have one visible home plus decomposition where it adds
information. It leaves `/stats`, savings math, cache attribution, and
the hero proxy savings card unchanged.

Refs #960

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- Removed redundant non-hero session-view captions that restated
proxy-compression token counts without adding a new dimension.
- Kept canonical homes for proxy compression and token usage details.
- Preserved Performance range context while avoiding adjacent
restatement of hero averages.
- Added a static dashboard regression for repeated session metrics.

## Testing

- [x] Unit tests pass (`uv run pytest
tests/test_proxy_dashboard_stats_cache.py -q`)
- [x] Linting passes (`uv run ruff check
tests/test_proxy_dashboard_stats_cache.py`)
- [ ] Type checking passes (`uv run mypy headroom`)
- [x] New tests added for new functionality
- [x] Manual testing performed

### Test Output

```text
$ uv run pytest tests/test_proxy_dashboard_stats_cache.py -q
12 passed, 1 skipped, 1 warning in 19.24s

$ uv run ruff check tests/test_proxy_dashboard_stats_cache.py
All checks passed!
```

## Real Behavior Proof

- Environment: Windows, Python environment from `uv sync --extra dev`,
browserless dashboard HTML inspection.
- Exact command / steps: load `get_dashboard_html()` in the focused
dashboard stats test and assert removed duplicate captions stay removed
while canonical metric owners remain present.
- Observed result: session-view repeated savings and performance labels
no longer duplicate the same numbers without context.
- Not tested: full browser screenshot and history-view de-duplication.

## 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] 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 or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [x] I have updated the CHANGELOG.md if applicable

## Additional Notes

No `CHANGELOG.md` edit: this repo generates changelog entries from
conventional commits. This intentionally avoids the hero proxy savings
card already covered by #927 and #1649, and it does not fold provider
cache discount into Headroom-value savings.

headroom/dashboard/templates/dashboard.html

@@ -433,7 +433,6 @@
                     <div>
                         <div class="text-xs text-gray-500 mb-1">Compression</div>
                         <div class="text-2xl font-light tabular-nums text-emerald-400" x-text="'$' + formatCurrency(stats.cost?.compression_savings_usd || 0)"></div>
-                        <div class="text-xs text-gray-500" x-text="formatNumber(stats.tokens?.proxy_compression_saved || 0) + ' proxy tokens removed'"></div>
                     </div>
                     <template x-if="(stats.cost?.cache_savings_usd || 0) > 0">
                         <div>
@@ -1079,18 +1078,10 @@
             <div class="bg-surface rounded-lg p-4 border border-border">
                 <div class="text-sm font-medium mb-4 text-gray-300">Performance</div>
                 <div class="space-y-3">
-                    <div class="flex justify-between items-center">
-                        <span class="text-sm text-gray-400">Headroom Overhead</span>
-                        <span class="font-mono text-sm" x-text="(stats.overhead?.average_ms || 0).toFixed(0) + 'ms avg'"></span>
-                    </div>
                     <div class="flex justify-between items-center">
                         <span class="text-sm text-gray-400">Overhead Range</span>
                         <span class="font-mono text-sm" x-text="(stats.overhead?.min_ms || 0).toFixed(0) + ' - ' + (stats.overhead?.max_ms || 0).toFixed(0) + 'ms'"></span>
                     </div>
-                    <div class="flex justify-between items-center">
-                        <span class="text-sm text-gray-400">TTFB (upstream)</span>
-                        <span class="font-mono text-sm" x-text="((stats.ttfb?.average_ms || 0) / 1000).toFixed(2) + 's avg'"></span>
-                    </div>
                     <div class="flex justify-between items-center">
                         <span class="text-sm text-gray-400">TTFB Range</span>
                         <span class="font-mono text-sm" x-text="((stats.ttfb?.min_ms || 0) / 1000).toFixed(2) + ' - ' + ((stats.ttfb?.max_ms || 0) / 1000).toFixed(2) + 's'"></span>

tests/test_proxy_dashboard_stats_cache.py

@@ -605,6 +605,17 @@ def test_dashboard_uses_cached_stats_and_lazy_history_feed_polling() -> None:
     assert "cliFilteringLabel + ' Filtered (lifetime)'" in html
 
 
+def test_dashboard_session_metrics_do_not_repeat_proxy_tokens_without_new_context() -> None:
+    html = get_dashboard_html()
+
+    assert "proxy tokens removed" not in html
+    assert '<span class="text-sm text-gray-400">Headroom Overhead</span>' not in html
+    assert '<span class="text-sm text-gray-400">TTFB (upstream)</span>' not in html
+    assert "Overhead Range" in html
+    assert "TTFB Range" in html
+    assert "Proxy Removed" in html
+
+
 def test_proxy_throughput_in_stats_endpoint(monkeypatch: pytest.MonkeyPatch) -> None:
     """Verify that the /stats endpoint includes a 'throughput' key in the response.