perf(expect): avoid redefining matcher state

[scttcper]

Codex assisted with this PR; I reviewed the changes and benchmark results.

Every expect(value) updates assertionCalls, but setState was redefining every existing state property and writing the same object back to its WeakMap. This applies only the incoming patch descriptors and inserts state into the map only during initialization, while preserving getters such as testPath.

expect(value)	Calls/sec	Relative
Before	606,431	1.00x
After	2,042,637	3.37x

Measured on Node 26.7.0 with 10 samples of 250,000 calls after a 50,000-call warmup.

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[scttcper]

i'm a human watching codex/chatgpt profile a few things. found this small change to expect that should hopefully be pretty straight forward and i have reviewed. Very much a microbenchmark but should be small enough change to go with it.

edit: failing test looks like a flake

[sheremet-va]

LGTM 👍