feat(storage): add schema downgrade guard

[luoye520ww]

Problem

Older application versions must not silently write data created by a newer schema, while users still need a safe read-only view and export path.

Scope

This PR adds only a pure schema compatibility decision contract and tests. It does not change persisted formats, migrations, startup wiring, or UI.

Changes

• Allow read/write only when stored and supported versions match.
• Force newer data into read-only mode with export available.
• Require migration before writing older data.
• Fail closed for invalid versions, null input, and unknown fields.

Validation

pm.cmd exec vitest run src/main/services/schema-downgrade-guard.test.ts (3 passed)

pm.cmd run typecheck (passed)

pm.cmd --prefix kun run typecheck (passed)

pm.cmd run lint (passed)

pm.cmd run build (passed)

• git diff --check (passed)

Review

Completed functional, lifecycle, data integrity, security, cross-platform, compatibility, test quality, and scope review. The guard never mutates persisted data.

Follow-ups

Startup read-only mode, export wiring, and migration execution belong in separate PRs.

[luoye520ww]

This work is consolidated into #942, which combines the Thread Store Doctor, bounded backups, and schema write guard with one tested scope. Closing the duplicate to keep review focused.