Centralize the pinned Parsar contract baseline

[sam2tom]

Outcome

Define one machine-readable source for the pinned Parsar contract revision used by Web.

Current behavior

The same Parsar revision is repeated in runtime boundary copy, Environment UI, bilingual README files, protocol documentation, E2E expectations, and fixture paths. The values currently agree, but manual updates can drift.

Acceptance criteria

• Add one machine-readable current contract-baseline declaration.
• Derive or validate runtime copy and current-baseline documentation against that declaration.
• Add a deterministic repository check that fails on inconsistent current-baseline references.
• Keep historical protocol fixtures immutable and explicitly exempt them from being rewritten to the newest revision.
• Document the update procedure when Parsar advances.

Non-goals

• Pulling Parsar during a browser build.
• Automatically accepting a new contract revision.
• Rewriting historical fixtures.