Disambiguate hash collisions in internal/hashmap via Hasher.Equal

[PratikDhanave]

Problem

internal/hashmap's Hasher interface requires an Equal(T, T) bool method, and Map stores each entry's key — but Load, Delete and Set keyed purely on the 64-bit hash and never called Equal. The stored key and the Equal method were dead unless used for collision disambiguation.

When two distinct keys hash to the same value:

m.Set("alice", "a")
m.Set("bob", "b")   // same hash → overwrites alice's entry
m.Load("alice")       // → ("b", true)   WRONG value
m.Load("carol")       // → ("b", true)   returns a value for a key never stored
m.Len()                // → 1              silent data loss

Delete similarly removed whatever entry shared the hash, not the one whose key actually matched.

The hashers in use (e.g. the workflow scope/update-key hashers) are maphash-based, so collisions are rare — but the map is a general-purpose keyed container and the Equal contract exists precisely to make it correct regardless.

Fix

Store entries in per-hash buckets (map[uint64][]entry) and use Hasher.Equal to select the matching entry in Load, Delete and Set. This honors the interface contract that the stored keys and Equal method were already there to support. No exported method signature changes. (Also fixes a th//e comment typo.)

Test

Adds the package's first test, TestMap_DistinctKeysWithCollidingHashCoexist, using a hasher that forces all keys to one hash. It asserts distinct colliding keys coexist, a never-stored colliding key resolves to (zero, false), and deleting one colliding key leaves the other. Fails on main (wrong values, Len()==1), passes with the fix. All existing workflow/checkpoint/state callers still pass.

[Copilot]

Pull request overview

This PR fixes correctness issues in internal/hashmap where Load, Set, and Delete previously keyed only on the 64-bit hash and did not use Hasher.Equal, causing silent data corruption on hash collisions. It changes the internal storage to per-hash buckets and adds a focused regression test that forces collisions to verify correct behavior.

Changes:

• Store entries as map[uint64][]entry to support multiple keys per hash.
• Use Hasher.Equal to disambiguate collisions in Load, Set, and Delete (and update iterators/String accordingly).
• Add TestMap_DistinctKeysWithCollidingHashCoexist to validate collision correctness.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
internal/hashmap/hashmap.go	Switches to per-hash buckets and uses Equal to correctly handle hash collisions across map operations.
internal/hashmap/hashmap_test.go	Adds a regression test using a forced-collision hasher to ensure distinct colliding keys coexist and delete/load behave correctly.

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[PratikDhanave]

Done — added an entry count updated in Set/Delete/Clear so Len() is O(1) again, and Delete now shifts the tail down and zeroes the vacated slot to avoid retaining the removed key/value.