Contentious constructs: rule registry, secondary recall metric, and failures-view feedback

[LucaCappelletti94]

Adds a contentious-constructs layer. A contentious construct is one the reference engine accepts but a parser may reasonably reject, like the SQLite TCL bind variables ($::x) and the duplicate target columns from lemon-rs#102 and #89. Strict, oracle-graded recall stays the headline number and is untouched. Rules are one TOML file each under contentious/ (tcl-variables, duplicate-target-columns, sqlite-nonstandard-alter), matched against a masked form or a named structural predicate, with a guard test keeping each honest, so adding one is a data-only change.

bench.json gains recall_excl_contentious_pct, shown as a grey sub-line under strict recall. On SQLite, sqlite3-parser is 99.78% strict and 100% excluding contentious, so its whole gap is these constructs. The failures view gains per-row buttons linking to three prefilled issue forms, and a row already tagged contentious shows a badge plus a dispute button. CONTRIBUTING.md covers adding rules.

[codecov]

Codecov Report

❌ Patch coverage is 79.72973% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.23%. Comparing base (dcd509a) to head (cb1e32f).

Files with missing lines	Patch %	Lines
src/export.rs	27.27%	16 Missing ⚠️
src/contentious.rs	89.83%	12 Missing ⚠️
src/report.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   44.53%   46.23%   +1.69%     
==========================================
  Files          28       29       +1     
  Lines        3460     3608     +148     
==========================================
+ Hits         1541     1668     +127     
- Misses       1919     1940      +21     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.