fix(code): quarantine Perl parser from code-aware compression (#2204)

## Description

`headroom proxy` can wedge when code-aware compression enters the
tree-sitter Perl external scanner and the native scan keeps the GIL
indefinitely. Current main still has two routes into that scanner:
explicit `perl` or `pl` hints flow through `CodeAwareCompressor`, and
`detect_language()` can nominate Perl on non-Perl code because its
prefilter matches generic sigils such as decorators, JSDoc tags, and
shell variables before phase 2 parses every surviving candidate grammar.

This change quarantines Perl at the code-aware compression funnel
without widening scope. Perl remains recognized at the input boundary,
but live proxy compression no longer requests a Perl parser. Non-Perl
code keeps its existing code-aware behavior. Real Perl falls back
through the existing safe Kompress or passthrough contract instead of
entering tree-sitter. The diff stays inside
`headroom/transforms/code_compressor.py` plus focused parser-safety
regressions.

Refs #2185

## 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

- Added a Perl quarantine list in
`headroom/transforms/code_compressor.py` and used it to stop Perl
candidate parsing during language detection.
- Added a hard `_get_parser()` guard so no live code-aware path can
construct a Perl parser.
- Routed resolved explicit Perl hints through the existing safe fallback
or passthrough contract before AST compression.
- Added focused parser-safety regressions for non-Perl candidate bleed,
explicit `perl` and `pl`, inferred real Perl, mixed fenced Perl content,
fallback-disabled passthrough, and non-Perl negative space.

## Testing

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

### Test Output

```text
uv run pytest tests/test_perl_scanner_safety.py -q
8 passed in 1.93s
uv run ruff check headroom/transforms/code_compressor.py tests/test_perl_scanner_safety.py
All checks passed!
uv run ruff format headroom/transforms/code_compressor.py tests/test_perl_scanner_safety.py --check
2 files already formatted
```

## Real Behavior Proof

- Environment: Windows, Python from the synced `uv` environment, `dev`
and `code` extras installed, no provider call
- Exact command / steps: Run `uv run pytest
tests/test_perl_scanner_safety.py -q`.
- Observed result: `8 passed in 1.93s`; the suite proves explicit `perl`
and `pl`, inferred real Perl, mixed fenced Perl, and non-Perl
negative-space routes all avoid Perl parser entry.
- Not tested: the reporter's macOS payload and long-running concurrent
workload

## 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
- [ ] 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 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

- Use `Refs #2185`, not `Closes #2185`. The wedge surface is this PR's
scope, but #2185 also carries a separate orphaned `headroom mcp serve`
report that this slice does not address.
- This is a reachability fix. It does not repair the upstream Perl
scanner and it does not harden other grammars against the same class of
native wedge.
- `CHANGELOG.md` remains unchanged because Headroom generates release
notes from conventional commits.
- Merged PR https://github.com/headroomlabs-ai/headroom/pull/2114
addresses a different cooperative compression stall and stays separate
from this native parser-entry slice.

---------

Co-authored-by: JerrettDavis <mxjerrett@gmail.com>

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