Reject invalid Unicode escapes before runtime evidence joins #515

🤖 Generated by the Agentic Engineer

Problem and evidence

Runtime JSON observations retain literal source bytes but do not validate escaped Unicode before decoding. An offline classification replay repairs an invalid escaped character into the same identity as a different successful workflow; the resulting classification omits the failed workflow. The unresolved-thread collector also accepts the repaired identity.

Expected outcome

Invalid escaped Unicode cannot establish a runtime observation or join identity. Preserve valid Unicode and return UNKNOWN for unsupported source representations before semantic classification.

Acceptance criteria

• Record a failing-workflow identity collision before the repair and a distinct valid-Unicode control.
• Validate escaped surrogate pairing before JSON decoding.
• Refuse malformed identities through both workflow and review-thread consumers.
• Preserve paginated input, decoded-key uniqueness, literal-byte preservation and producer failure checks.
• Exercise the installed helper route and its regression suites.

Rough size: S. Part of #223.