Close Kimi review gaps

[demirelo]

Implements the detailed Kimi repository review findings:

• adds signed R0 statement bundle and CI digest attestation
• adds source-level public theorem manifest completeness checks
• adds methodology-event schema/coverage validation
• adds R0/R1/R2 Lean signature bindings and shared protected axiom audit
• expands exact/binary64 reproductions for signed branches, identities, smallest grids, and R2/R3 witnesses
• updates README hydration guidance, methodology, theorem inventory, and source map

Verification: lake build --wfail; all 35 Lean acceptance modules; Python contract, manifest, methodology, and reproduction tests.