[P1] Harden artifact audit against complete-but-weak HTML

[ramsani]

Harden the deterministic artifact audit so HTML that is structurally complete but semantically weak gets blocked earlier.

Scope: update scripts/audit-artifact.py with minimal semantic checks, add at least one bad fixture such as complete-but-weak.html, and document the failure mode in ANTI_SLOP.

No-scope: no broad scoring rewrite, no UI redesign, no external dependencies, no deploy/infra/secrets/production changes.

Acceptance: audit blocks artifacts with generic job-to-be-done, evidence that does not affect recommendation, and copy/export output that is not runnable or reusable.

Rollback: revert script, fixture, and docs changes; existing golden validation returns to previous behavior.

[ramsani]

PR mínimo: actualizar scripts/audit-artifact.py, agregar fixture bad/complete-but-weak.html y documentar el riesgo en docs/ANTI_SLOP.md; validar con scripts/validate-golden-artifacts.sh y scripts/validate-runtime.sh; rollback: revertir script, fixture y docs.