feat(calibration): pure per-repo corpus slicing and labeled-density stats

[kai392]

Summary

• Add packages/loopover-engine/src/calibration/repo-corpus.ts (Epic: calibration expansion — self-correcting configuration and measured judgment #8211 track B's first building block) with two pure functions beside buildBacktestCorpus, per the epic's file-per-concern discipline:
  • sliceCorpusByRepo(cases) — deterministic Map<repoFullName, BacktestCase[]> keyed by everything before targetKey's last # (so an embedded # in a repo name never truncates it); unparseable keys (no #, or nothing before it) are dropped, never guessed; per-slice case order and first-appearance repo order preserved.
  • computeRepoCorpusDensity(cases, minVisible, minHeldOut, heldOutFraction, splitSeed) — per-repo {cases, confirmed, reversed, eligible} where eligible re-runs the deterministic seeded split on that repo's OWN slice and requires both sides to clear the floors — the exact never-on-noise bar evaluateKnobLoosening/evaluateKnobDrift apply globally.
• Aggregate-only shapes throughout (repo names + numbers — no target keys, no metadata), pinned by an invariant test.
• Both functions exported from the engine barrel, per the deliverable.

Closes #8215

Test plan

• test/unit/repo-corpus-engine.test.ts (vitest, importing the engine source — the root mirror suite per the engine blind-spot rule): multi-repo slicing with order preservation; embedded-# repo keying; unparseable-key drops; empty corpus; density floors above/below (including a held-out-side-only miss); split determinism per slice; the no-corpus-content invariant
• packages/loopover-engine/test/repo-corpus.test.ts (node:test vs dist): barrel re-export + slicing/density round-trip
• Local coverage on the new file: 19/19 lines, 8/8 branches; engine workspace suite 663 green; npm run typecheck green
• CI validate

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.10%. Comparing base (0c8d3d6) to head (b3cfabf).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8234      +/-   ##
==========================================
- Coverage   92.10%   92.10%   -0.01%     
==========================================
  Files         776      777       +1     
  Lines       78328    78348      +20     
  Branches    23668    23672       +4     
==========================================
+ Hits        72147    72163      +16     
  Misses       5062     5062              
- Partials     1119     1123       +4     

Flag	Coverage Δ
shard-1	57.49% <0.00%> (+2.56%)	⬆️
shard-2	51.82% <0.00%> (-0.78%)	⬇️
shard-3	53.17% <100.00%> (-1.59%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ges/loopover-engine/src/calibration/repo-corpus.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[kai392]

Withdrawing: #8233 (filed earlier) merged and closed #8215 — this duplicate is superseded.