Export checkpoint recordings and replay verified frames offline

[marpaia]

Periodic checkpoints can now be exported in explicit order with microsimulator export-replay, then opened as a recording folder in the standalone viewer. Timeline seeking, backward/forward stepping, play/pause and frame-rate controls display exact snapshots and physical timestamps while preserving camera, reference-grid and channel preferences.

The versioned manifest records per-frame byte lengths/digests, source-checkpoint digests and source backend separately from CPU export provenance. Export never runs model code. The reader validates safe local references and scene integrity on demand; a bounded LRU cache and a single decode worker prevent history growth and stale seek results. Missing or damaged frames identify their ordinal/path and retain the last valid view. Cache limits cover cached decoded frames, not total browser heap or the active renderer.

Validation: 344 Python tests passed (56 environment-dependent skips); 66 viewer tests; typing/lint/build checks. Chromium exercised exported growth/division/removal, stable-ID selection, reverse stepping, playback/end handling, rapid-seek races, signal-grid changes, labels, camera/grid preferences, malformed-frame recovery, keyboard controls and the supported 880px layout. Combined fixed-scale/composite/device acceptance passed in Chromium. CUDA provenance tests are data-only and do not claim CUDA execution.

Originally reviewed against frozen marpaia/base-16, combining #20 and #18. Prerequisites were integrated before retargeting to master.

Closes #16.

A follow-up fixes Play while a seek is pending or has failed: it preserves the requested position and retries failed frames instead of advancing from the last displayed frame. Six regressions failed before the correction and pass afterward; the isolated viewer suite now has 72 passing tests, and the combined Chromium replay check exercises the pending-seek interaction.

Independent review is complete against the original acceptance criteria and intended review base. The review report records per-criterion evidence, both corrected campaign findings, exact tested heads, landing order and platform limits.

Merged into master in the approved campaign order: #26, #27, #28, #31, #34, #36, #29, #30, #33, #35, #37, #32. The final tree preserves the reviewed implementation and integration regressions. Prerequisites were integrated with merge commits, and the frozen review bases were retained.

Landing validation: 534 Python tests passed (57 environment-dependent skips), 115 viewer tests passed, Ruff lint, strict type checks, viewer build/format checks, and actual Chromium feature/integration checks passed. Pyright retains the existing native-extension source warning. A Windows-only documentation link-test encoding failure was reproduced and fixed with an explicit UTF-8 read; the corrected final PR heads passed Windows CLI/shutdown checks and CUDA compilation. NVIDIA runtime execution remains unverified.