fix(combineSlices): scope stateProxyMap per instance to prevent cross-instance proxy collisions

[JSap0914]

Problem

combineSlices.ts declares stateProxyMap at module level:

const stateProxyMap = new WeakMap<object, object>()

This WeakMap is shared across all combineSlices() instances. The module-level noopReducer always returns the same emptyObject constant as initial state when no slices are given, so two empty combineSlices() calls both produce the same object reference as their initial state.

When a selector from instance A is called first with that shared state, a proxy is cached in the module-level stateProxyMap keyed by the state reference. A subsequent selector call from instance B with the same state reference retrieves the cached proxy from instance A, which closes over instance A's reducerMap and initialStateCache — so instance B's injected reducer's initial value is never consulted and the wrong value is returned.

const reducer1 = combineSlices()
const reducer2 = combineSlices()

// Both return the same  reference (noopReducer)
const preState = reducer1(undefined, { type: '@@INIT' })

reducer1.inject({ reducerPath: 'x', reducer: () => 'A' })
reducer2.inject({ reducerPath: 'x', reducer: () => 'B' })

const sel1 = reducer1.selector(s => s.x)
const sel2 = reducer2.selector(s => s.x)

sel1(preState) // 'A'  ✓
sel2(preState) // 'A'  ✗  — should be 'B'

Fix

Move stateProxyMap inside combineSlices (one WeakMap per instance) and thread it as an explicit parameter through createStateProxy. Each instance now has an isolated proxy cache, so selecting against a shared state object from a different instance creates a fresh proxy with the correct reducerMap and initialStateCache.

Testing

A new unit test in combineSlices.test.ts reproduces the bug (fails without this fix, passes with it). All 88 existing test files continue to pass.

AI-assisted fix

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commit: d5b5136

[EskiMojo14]

seems edge case-y, but nonetheless a legitimate bug with a simple fix - thanks! (and thank you for disclosing that you're using AI for these, the honesty is appreciated :P)