Stream framed checkpoints from the shared engine

[chaoz23]

Summary

• adds a runtime-branded, callback-scoped shared-engine checkpoint source and writes framed-v2 checkpoints directly from the paused engine
• keeps the logical checkpoint schema and stable ID unchanged while avoiding a materialized checkpoint graph and monolithic checkpoint JSON on the source-native write path
• hardens source lifetime, mutable-callback isolation, structural restore, bounded framed reuse verification, effective read limits, file-descriptor teardown, and Windows recovery-slot contention
• records a preregistered matched six-cell checkpoint-v2 proof on Heresy II and The Intercept

Evidence

• implementation commit: 9312811be8f0e79b8f05ce7e0d09a2626063b8ea
• proof-record commit: 416305c7a49e0670914dcc9116005890616c3e4a
• proof result SHA-256: 0443af81a3aa3312101ccbbf14161e7162d3332a613e14b5d27cbb94ee880088
• full test suite: 410 total / 407 passed / 0 failed / 3 expected skips
• package dry run: 122 entries, version 0.7.2
• Heresy II: exact 100,000 → 150,000 split/resume parity for legacy-v1 and framed-v2
• The Intercept: exact framed-v2 600,000 → 650,000 split/resume parity; legacy-v1 readback reaches the typed, non-destructive 536,870,888-byte decompression boundary
• both framed proof writes report zero materialized checkpoint graphs and preserve the existing logical checkpoint identity

Scope

• legacy-v1 remains the default write format
• no release, allocation/retention policy promotion, or checkpoint-format default change
• the proof supports exact resume, zero-materialized-graph source writing, a typed legacy readback boundary, and artifact preservation only
• it does not claim universal performance or portable memory improvement, broader story coverage, or an InkBench improvement
• Compact shared checkpoints before frontier artifacts dominate campaigns #156 and [long-run] Add versioned retained-resource and semantic-yield interval telemetry #216 remain open and partial

Related: #156, #215, #216