refactor(storage): add bounded context offload lifecycle

[likun666661]

Summary

• Extend the context-offload authority with atomic reference copy, Session retirement, and bounded garbage collection.
• Add schema v2 with an indexed context_gc_candidates eligibility set, so GC selects bounded metadata batches instead of scanning or anti-joining the complete blob population.
• Keep logical and physical usage counters transactionally consistent across copy, release, retirement, shared blobs, and GC.
• Add v1-to-v2 orphan migration, failpoint rollback coverage, and EXPLAIN QUERY PLAN assertions for the Session and GC indexes.

This is a stacked, storage-only follow-up to #4078. Its base is refactor/context-offload-sqlite-foundation; it should be rebased to main after #4078 merges. No runtime consumer is wired yet, so this does not change production behavior.

Refs #4071
Depends on #4078

Bounded-work contracts

• Session retirement starts from context_refs_session and is proportional to references owned by that Session.
• Last-reference release records eligibility transactionally in context_gc_candidates.
• GC reads at most maxBlobs + 1 indexed candidate rows, obeys both blob and byte budgets, and never materializes payloads while selecting work.
• Copy points new Session-owned refs at existing immutable blobs; physical usage does not increase.

Verification

• npm --workspace @maka/core run build
• npm --workspace @maka/storage run build
• NODE_NO_WARNINGS=1 node --test packages/storage/dist/__tests__/sqlite-context-offload-store.test.js — 16/16 pass
• NODE_NO_WARNINGS=1 node --test --test-reporter=dot "packages/storage/dist/**/*.test.js" — full storage suite passes
• npx biome check on all changed TypeScript files
• npm run check:asf-headers

AI use

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool and scope: OpenAI Codex contributed repository inspection, lifecycle and schema design, implementation, tests, and PR text. The commit includes the required Generated-by trailer.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes
• No — storage contracts remain unwired to production consumers

[Astro-Han]

Approved.