fix(shared): fix deepMerge data-corruption bugs and add tests

[kridaydave]

Fixes two deepMerge bugs: it destroyed Date/Map/class-instance values (prototype loss) and silently discarded the entire base on a primitive patch.

• Only recurse into plain objects now; Date/Map/instances are overwritten whole.
• Throw on a primitive patch instead of dropping the base (null still allowed).
• Added regression tests in Struct.test.ts and Struct.edgeCases.test.ts.

All gates green (typecheck, check, test).

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Note

Medium Risk
Behavior change for invalid primitive patches (now throws) and for non-plain-object values during merge; shared utility so callers relying on old Effect isObject recursion could see different merged shapes.

Overview
Fixes data-corruption bugs in shared deepMerge by only recursing when both the existing value and patch value are plain objects (Object.prototype), via a new isPlainObject helper. Date, Map, class instances, arrays, and functions are now replaced whole instead of being field-merged like POJOs.

Error handling for invalid patches is tightened: a non-null primitive patch (e.g. a number) now throws instead of silently returning a corrupted result; null patch is still allowed.

Adds Struct.test.ts for normal merge behavior (immutability, nesting, undefined skips) and Struct.edgeCases.test.ts to lock in the current contract (array overwrite, null vs undefined, Date/functions, primitive patch throw).

^{Reviewed by Cursor Bugbot for commit 966cd6b. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix data-corruption bugs in deepMerge and add test coverage

• Introduces isPlainObject in Struct.ts to restrict recursion to plain objects; dates, maps, and class instances are now overwritten rather than recursively merged.
• deepMerge now throws when the top-level patch is a non-null primitive (e.g. number, array, function) instead of silently returning it.
• Adds primary tests in Struct.test.ts and edge-case tests in Struct.edgeCases.test.ts documenting both corrected and remaining lossy behaviors.
• Behavioral Change: callers passing a primitive or array as the top-level patch will now receive a thrown error instead of a silent no-op overwrite.

^{Macroscope summarized 966cd6b.}

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Approvability

Verdict: Needs human review

This bug fix introduces meaningful runtime behavior changes to a shared deepMerge utility: new exception pathways for primitive patches and changed merge semantics for Date/Map/class instances. These changes to shared infrastructure warrant human review to ensure callers aren't affected.

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