Preserve supervisor poison failures and register worker ownership

[sorenbs]

A panicking task builder poisons the registration lock after its task ID is assigned. Readiness, phase snapshots and subsequent registration must continue to fail rather than conceal that partial operation. Four method-scoped expectations now document those deliberate poison contracts; the new regression invokes a failing builder, checks every observation path and proves a later builder never runs.

The existing worker spawn has one statement-scoped expectation and an explicit source-owner registration. Its handle remains retained under the phase lock before shutdown can drain the task map. No production function body changes or poison-recovery fallback are introduced.

Completed local validation: common quality (12 syntax tests and 47 Python controls), all 15 task tests, 52 actual-source invariant tests with 1,024 property cases and 11 service invariant/Loom tests. All 13 exact-head CI checks pass, including the configured compiler, compatible Miri/corpus and actual-diff mutation gates. The changed source contains only explicit diagnostic ownership and the new fixture; the prerequisite cancellation mutation and compatible buffer checks already passed on #64, and are not relabeled as new local runs here.

Targets slate; includes #64 and its prerequisites. Addresses five additional inherited warning occurrences, clears the remaining warnings in src/tasks.rs, and prunes both active platform inventories plus the replaced legacy spawn entry. Shutdown-driver warnings in src/tasks/shutdown.rs remain separate work. Immutable adoption inventories are unchanged.