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  perf/is-plain-object-constructor-check
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fix(router-core): keep records with a constructor key plain `?constructor=foo` decodes to an own `constructor` data key on the null-prototype search record, and `(o.constructor ?? Object) === Object` read that key. Such records (and the `{ ...search }` copies of them) were classified as non-plain, so `replaceEqualDeep` handed back `next` without sharing and `deepEqual` reported them unequal. Keep the constructor comparison as the fast accept: it is an inline-cache load for literals and null-prototype records and is what the PR's speedup comes from. When `constructor` is anything else, consult the actual prototype instead of rejecting: `null` or one whose `constructor` is `Object` means a record, anything else a class instance. The prototype lookup is a builtin call (~6ns in megamorphic optimized code), so running it unconditionally would return most of the gain: median hz over 3 rotated rounds, `replaceEqualDeep` equal empty 10.9k -> 8.6k (-21%), equal array of objects 470 -> 386 (-18%), `deepEqual` equal empty 58.0k -> 35.9k (-38%). With the fallback only on the mismatch path the hot cases stay within noise of the shortcut and the constructor-key cases now do the sharing work at the same cost as other null-prototype records. `react-router.minimal` gzip is +15 B against the shortcut and still 44 B below the `toString` implementation. Benchmarks the predicate directly (literal, null-prototype record, record with a constructor key, class instance) and sharing of a search record that carries a constructor key. Tests both record shapes and parser output, all `constructor` value types, and that a class instance with an own `constructor` key stays opaque. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

nx affected --targets=test:eslint,test:unit,test:e2e,test:types,test:build,build --parallel=3

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