fix(bindings): preserve reprojection identity

[chubes4]

Summary

• preserve projected binding anchor identity across canonical reprojection instead of trusting stale block indexes or offsets
• support provable partial bindings among identical unbound blocks
• fail closed when occurrence identity becomes ambiguous rather than silently retargeting providers

Verification

• composer test
• 275 parity fixtures
• fixture 20 and independent fixture plan validation
• inserted-identical, partial-binding, occurrence, and ambiguity contracts

Closes #873

AI assistance

OpenAI gpt-5.6-sol via OpenCode implemented and independently reviewed canonical binding identity, collision behavior, fail-close semantics, and tests. Chris Huber reviewed and owns every line.