chore: [P2] Add schemaVersion to graph.json with reject-and-fallback on mismatch

[codeakki]

Problem

Resolver/architecture changes will alter graph.json shape. We need a clean way for an older server to refuse a newer (or incompatible) on-disk graph instead of misreading it.

Proposal

Add a schemaVersion field to graph.json. On load, reject-and-fall-back-to-"no index" on mismatch (reuse the existing T4 schema-validation fallback path). Human-readable symbol IDs stay as-is (debuggability).

Acceptance

• schemaVersion written on index and checked on load
• Mismatch → graceful "no index" fallback, not a crash or misread (test included)