task layer: no introspection of scheduler decisions — a gated task_sched_trace seam

[InauguralPhysicist]

Found building eddy's interleaving explorer (eddy#6, fleet UI ladder rung 4; ledger entry F-EDDY-06).

The gap. A schedule visualizer wants "who ran when": run segments (resume → yield), the seeded ready-queue's pick order, and sleep/virtual-time wakeups. None of that is visible to .eigs code. eddy derives lanes from app-level records — op invoke/ok pairs on its logical clock plus explicit marks at each task_yield call site — which works only because eddy owns every yield site. A consumer that loads third-party task code, or wants to visualize task_sleep/virtual-clock scheduling (liferaft's cluster sim is the obvious second consumer), cannot hand-instrument every site. True run segments are simply not drawable today.

Proposal shape. A gated task_sched_trace seam, off by default like the observer/trace flags: when enabled, the runtime records {tick, resumed task id, cause: spawn|yield|sleep-wake|join-release|kill} into a history readable from .eigs (or emitted like --viz-events). Two hard constraints from the DST discipline:

• Pure reader: enabling it must not perturb the seeded pick sequence — a run with tracing on must be byte-identical to the same seed with it off (eddy's gate would assert this the way it asserts sink purity today).
• Tape-aware: records derive from the deterministic schedule, so they must NOT be tape N-records; a traced run under EIGS_REPLAY must reproduce them (the trace-tape engineer's Design: trace-tape format versioning — the tape is becoming a persisted artifact #411/tape-first review applies).

Two consumers waiting: eddy's explorer (upgrade derived lanes → true segments) and liferaft's visualizer (task-level view under the Raft sim). Not urgent — the derived view ships fine — but the surface is a real blind spot the fleet UI ladder keeps walking past.