feat: freeze Obsidian project context v4 contracts

[NeoMei]

Summary

• freeze the Obsidian project-context v4 schemas and strict Go/TypeScript wire validators
• add closed-loop session detail, problem-map, pricing, annotation, and complete session-index contracts
• preserve migration compatibility and pricing supersession integrity
• align every persisted integer to the JavaScript safe range and close public TypeScript enums
• reject lossy raw integer literals before JSON.parse can round them

Local verification

• go test -p 1 -timeout 5m -count=1 ./...
• go vet ./...
• go mod tidy -diff
• obsidian-plugin npm run check: 17/17 files, 128/128 tests, production build
• Go/plugin fixture parity: 21/21 pairs
• zero-token Gate A: 154/154 terminal, 151 indexed, zero model tokens
• zero-token Gate B publication/idempotence: passed
• independent review: no Critical, Important, or Minor findings

Acceptance boundary

Local Gate 0 is complete. Native Windows CI is still required before merge. This PR freezes the implementation contracts and does not claim that the six user-facing Obsidian feature plans are implemented.