Isolate alias-observer test counts across parallel fixtures

[tamashi095]

Parallel alias-observer fixtures shared a process-wide callback counter, so the resident-meter matrix could inflate the serialized fixture’s expected three callbacks. PR #723 passed qualification, but main run 34490147028 exposed the race (12 observed).

Store the test count per thread beside its existing per-thread sample capture. A deterministic two-thread regression makes two versus five actual observer calls and checks independent counts/captures after both finish. Production code and the qualified AudioWorklet pin are unchanged.

Validation: independent Astra XHIGH SOURCE PASS; deterministic old-counter mutation failed with combined count seven; full locked compiler library suites passed 50 tests each in debug and release-unwind; strict all-target/all-feature Clippy and formatting passed. The initial positive verification invocation reused the mutant cache and was invalid; that receipt is preserved, and the valid positive runs used a separate clean target.

Refs #724; unblocks #709 and #722 after corrected-main qualification. The original failed run and #714’s superseded disposition remain preserved.

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