fix(engine): dedup provider-track-record aggregation per provider+target

[jeffrey701]

What

computeProviderTrackRecords
(packages/loopover-engine/src/calibration/provider-track-record.ts) builds a stancesByTarget
map that correctly collapses multiple same-provider signals for one targetKey to the latest vote,
but the main aggregation loop then iterated the raw signals array — incrementing
signals/decided/shared/consensus/agreed once per raw row rather than once per distinct
(provider, targetKey) pair. Since the real caller loadLiveProviderTrackRecords reads raw
audit-event rows with no dedup, a provider re-reviewing the same PR (common after a new push) had its
precision and agreement stats inflated proportionally to how many times it re-reviewed.

Change

Collapse repeated (provider, targetKey) signals to the latest row before aggregating (a
Map keyed by provider + targetKey, keeping the last-seen row — matching stancesByTarget's own
latest-vote .set dedup), then iterate that deduped set. One distinct pair counts once, scored on
its latest vote.

Validation

• New test in test/unit/provider-track-record-engine.test.ts (imports the engine source, so Codecov
  grades it): a provider with two rows for the same target (pass then a re-review fail) now
  produces signals: 1 scored on the latest fail vote — was signals: 2 / agreementRate: 0.5
  before. Existing multi-provider consensus/split/precision assertions are unaffected.
• npx vitest run test/unit/provider-track-record-engine.test.ts → 8/8 pass; 100% line+branch
  coverage on every changed line.

Closes #8876

[superagent-security]

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

[jeffrey701]

Closing as a duplicate of #8954 (opened ~70s earlier for the same #8876 fix). Deferring to that PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.86%. Comparing base (32ee72e) to head (ab729bb).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8955      +/-   ##
==========================================
+ Coverage   90.56%   93.86%   +3.30%     
==========================================
  Files          96      807     +711     
  Lines       22490    80334   +57844     
  Branches     3884    24343   +20459     
==========================================
+ Hits        20367    75408   +55041     
- Misses       1945     3561    +1616     
- Partials      178     1365    +1187     

Flag	Coverage Δ
backend	95.15% <100.00%> (?)

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

Files with missing lines	Coverage Δ
...er-engine/src/calibration/provider-track-record.ts	100.00% <100.00%> (ø)

... and 710 files with indirect coverage changes