health gate evidence: advisory unless bundle-bound

[moonweave]

The write_scope axis has a complete trust chain: the declaration is DSSE-signed before execution, the observation is a named bundle subject, and Depone verifies the subject binding before comparing. The code_health axis did not — its per-gate exit/log files are the producer's transcription of worktree-receipt fields, not bundle subjects, and nothing here read the source_receipt_sha256 they carry. So a verdict said "these bytes report exit 0", while its wording implied the same guarantee write_scope provides.

What changes

• The rollup records the evidence substrate per gate (bundle-bound vs producer-transcribed) and says so in its human-facing text.
• Structural, not cosmetic: while a gate's evidence is producer-transcribed, it cannot block handoff even if the repository declared block enforcement. A repo may not promote evidence the verifier cannot authenticate into a blocking verdict.
• When the producer supplies a bundle-bound artifact manifest, each gate artifact is checked against it before exit codes are re-derived, and declared enforcement applies again. New contract-invalid error codes cover a missing artifact, a digest mismatch, and a manifest that is not a bundle subject.
• Older evidence without the manifest keeps working and stays advisory — no fail-closed on runs sealed before this change, and no silent treatment of old evidence as bound.

witnessd's producer side (emitting the manifest and promoting it to a subject) lands separately, after this.

Suite: 470 tests, one known environment failure (the uv-shim contract checker).

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