Add framed checkpoint v2 codec and bounded readback

[chaoz23]

Summary

• Add an opt-in framed-v2 checkpoint artifact and manifest codec with bounded, incremental frame readback; per-frame, per-component, and terminal-index integrity checks; exact EOF validation; typed byte/count/depth limits; and prompt cancellation of stalled I/O.
• Keep the logical SharedSearchCheckpoint at schema v1 and preserve the existing checkpoint-ID algorithm across layouts. The default writer remains legacy-v1.
• Split the logical checkpoint into eight fixed-order components and reassemble the exact checkpoint without changing deterministic frontier order, findings, witnesses, or split/resume results.
• Make same-ID legacy/framed writers converge on exactly one verified public layout through a neutral no-clobber publication path, including crash recovery, mixed-format races, byte-identical distinct-inode writers, three-writer neutral reuse, and caller-limit mismatches.
• Extend list/show/capabilities with explicit artifact-schema discovery while keeping public projections numeric and privacy-safe.

Verification

• npm test: 386 total, 383 passed, 0 failed, 3 expected local socket skips.
• npm pack --dry-run --json --cache /private/tmp/inkcheck-v2-npm-cache: passed, 118 package entries, version unchanged at 0.7.2.
• Independent frozen-tree review: 18 paths, +5150/-149, exact tree 85dfe9eaa0163bef5a7776ac4b4aa3e2eedb8216, no unstaged or untracked files.
• Focused codec and integration coverage includes corruption/truncation, future schemas, bounds, cancellation, exact resume, component growth, retention, crash windows, cross-process publication, and public-shape privacy.

Deliberate scope

This is an opt-in checkpoint-storage foundation under #156 and a bounded prerequisite within #215/#216. It does not make framed v2 the default, activate allocation/eviction/compaction/stopping policy, change search ordering, publish a release, run InkBench, or claim a current-head performance improvement. Engine-native streaming from live search structures and the matched Intercept plus second-story promotion evidence remain separate work. All related issues remain open.

Related: #156, #215, #216