hir: class-expression capture-refresh edge cases — param defaults, multi-eval name-keying, top-level-block/arrow refresh owners (#6604 follow-up)

[proggeramlug]

Consolidating three related CodeRabbit findings raised on PRs #6647/#6650/#6653 (all anchored in the #6604 class-expression capture-refresh machinery, merged via #6633). Each is a plausible correctness edge case that the semver-driven fix did not need, but that real code can hit. Needs verification against the shipped implementation first — the #6604 work includes multi-eval isolation tests that may already cover parts; treat these as claims to confirm or refute, with a parity fixture per case:

1. Param-default class expressions are deliberately discarded (get_param_default self-truncates its entries). Claimed counterexample: function f(x, C = class { get() { return x } }) { x = 2; return C } — the default-evaluated class should observe the body's x = 2 (same parameter binding in Node). Verify perry's param-capture machinery doesn't already handle this via a different path.
2. Name-keyed snapshot can cross-contaminate multi-evaluation factories: with A = make("a"); B = make("b"); new A(), refresh entries keyed by the shared template name could backfill A's stale slots from B's later snapshot. The hir: class-expression capture snapshot stale for vars assigned after the class — semver Comparator TypeError kills pi-native at init #6604 fix's ctor-prologue rebind + multi-eval tests may already cover this — confirm with an assigned-AFTER-class multi-eval fixture specifically (the existing tests reportedly cover assigned-before).
3. Refresh-owner gaps: top-level block captures may strand entries (module-init refresh handling doesn't drain them); expression-bodied arrows may discard entries before later live-binding updates.

Each confirmed case: minimal fixture byte-identical to node + targeted fix keeping refreshes scoped to the evaluated class object (per-evaluation identity rather than shared-name keying, where required).

Refs: CodeRabbit inline comments on #6647 (lower_patterns.rs:1469, arm_class.rs:205), #6650 and #6653 (arm_class.rs:205).