fix(scoring): stop garbage upstream bodies from creating false "unmodeled constants" drift reports

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⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

src/scoring/model.ts's fail-closed guard (lines 78-87) correctly freezes the last-good snapshot when the upstream constants.py fetch returns HTTP 200 but the body is non-source garbage (recognizedCount < MIN_RECOGNIZED_SCORING_CONSTANTS) -- but only when a valid lastGood snapshot exists. If it doesn't (first run, or last-good was itself a fallback), execution falls through, and the drift-sync guard at line 137 is only if (constantsResult.ok) -- not a usable-content check -- so syncUnmodeledScoringConstantDrift still runs findUnmodeledUpstreamConstants against the garbage body and can open a spurious high/medium-severity upstream_drift_reports row.

Requirements

Change line 137's guard to check content usability (e.g. if (constantsUsable), mirroring whatever variable/check the fail-closed guard at line 78-87 uses to determine "usable" vs "garbage-but-200"), so a garbage-body fetch never feeds syncUnmodeledScoringConstantDrift.

Deliverables

• Line 137's guard checks content usability, not merely HTTP-ok, before running syncUnmodeledScoringConstantDrift
• A test asserting a 200-with-garbage-body upstream fetch (with no valid lastGood snapshot) does not produce a spurious upstream_drift_reports row

All of the above Deliverables are required in the same PR unless the deliverable text itself states otherwise.

Test Coverage Requirements

src/** -- 99%+ patch coverage, branch-counted, on the corrected guard.

Expected Outcome

A garbage-but-200 upstream constants.py fetch with no prior good snapshot no longer opens a spurious high/medium-severity drift report.

Links & Resources

• src/scoring/model.ts:78-87,137-142

[linear-code]

JSO-6181