test(runtime): let the dispatch-IC generation retry actually fire (#7365)

[proggeramlug]

One of #7365's two named flaky tests is fixed: obj_dispatch_ic_tests goes from 10 of 12 isolated runs failing to 0 of 20.

with_stable_gen exists because dispatch-IC entries are keyed on VTABLE_GEN, and any class registration anywhere retires the cache — including a_class_registration_invalidates_every_entry in the same module, which calls test_bump_vtable_generation() on purpose. So an insert/lookup pair can straddle a bump and miss for a reason that has nothing to do with what is being asserted. The helper was written to retry in exactly that case.

The retry could never fire. body asserted internally, so a straddled pair panicked on the setup assertion ("the entry we just inserted must be findable") before the loop got to re-check the generation. The 64-retry budget was never spent.

body now returns whether its observations were valid: false means "a bump landed mid-pair, nothing was learned", which is a retry rather than a failure. The assertions the tests exist for — a different name at the same address must miss, a different class id must miss, an over-long name must not alias a truncated key, a prefix must not hit — stay assertions.

Why this matters beyond one test, and why it looked so strange. The failure rate inverted with scope: running only this module put its five tests — including the deliberate bumper — on threads together and failed 10 of 12, while the full 2000-test suite spread them apart and failed about 1 in 6. So the standard triage move, re-running just the failing test, made an intermittent test look reliably broken, and running the suite made it look nearly fine. Both readings were wrong, and #7365 is largely a catalogue of exactly that confusion.

Measured after the fix: 0/20 isolated runs fail. The full suite still fails about 1 in 10, but on a different test — gc::tests::root_words::bare_address_in_global_root_survives_a_real_collection, a sibling of the bare_address_in_shadow_slot_… case already measured at 2/10 on clean main. That family is untouched here and remains #7365's other half; it is a GC-timing intermittency rather than a test-isolation one, so it needs a different fix.

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Reviewing files that changed from the base of the PR and between 1804991 and e6069be.

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• changelog.d/7822-dispatch-ic-retry-can-fire.md
• crates/perry-runtime/src/object/class_registry/dispatch.rs

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