docs: software spec, requirements register, and implementation plan
Adds the requirements baseline for the pipeline redesign: - SPEC.md — software requirements with Current/Gap deltas per item, so the document doubles as a work list. - requirements.md — stable flat IDs (AR/DP/IR/GR/VR) with parent traces, priorities, statuses, and a per-requirement verification plan. Replaces the thematic A1..E8 scheme, which had already produced an A1a and an out-of-order E6; IDs are now permanent and never reused. - IMPLEMENTATION-PLAN.md — phased work. The central change is AR-012: presence follows track extent rather than per-frame recognition, so a window starts when an actor appears rather than when the recogniser first succeeded. anneal_sec and extinction_sec are withdrawn rather than retuned — a track that survives its own gaps leaves them nothing to do. Verification is shaped by CI running on an N100 with no dGPU: the existing HDF5 dump makes everything downstream of embedding replayable on CPU, which covers the bulk of the redesign. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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