perf(router-core): faster enumerable-key collection in structural sharing (#8010)

* perf(router-core): faster enumerable-key collection in structural sharing

getEnumerableOwnKeys (used by replaceEqualDeep) called
Object.getOwnPropertyNames and then invoked propertyIsEnumerable once per
key to verify every own string prop is enumerable. Replace that O(n) loop
of JS method calls with Object.keys (native, enumerable-only) plus a
single length comparison against getOwnPropertyNames to detect
non-enumerable string props. Symbol handling is unchanged.

Behavior is identical (verified against the existing suite + fuzzing): the
function still returns the enumerable own keys in the same order and still
bails (returns false) for objects with any non-enumerable own string or
symbol property.

replaceEqualDeep runs on every selector result on every state update when
defaultStructuralSharing is enabled, so this is a hot client path for
selector-heavy apps. Measured ~1.3-1.5x faster on typical router state.

* style(router-core): address enumerable key review

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Co-authored-by: Yagiz Nizipli <yagiz@nizipli.com>