Summary

Fixes #36800 — a ref callback (and other values) failing to memoize "in all situations."

A conservatively-mutable value created before a hook call (e.g. const isChromatic = getIsChromatic() before useResizeDetector()), then read by a ref callback and captured by another expression, gets a mutable range that spans the hook. InferReactiveScopeVariables then unions the ref callback, the other consumer, and that value into a single reactive scope whose range straddles the hook. FlattenScopesWithHooksOrUse prunes any hook-spanning scope wholesale (scopes can't be conditional on a hook), so the ref callback is never memoized — producing a fresh ref identity every render and forcing React to detach/reattach the ref each render.

Fix

Enforce the existing invariant "a reactive scope can never span a hook/use call" during scope formation instead of only cleaning it up destructively afterwards:

• InferReactiveScopeVariables.findDisjointMutableValues — never union an instruction's operands across a preceding hook/use call. The instruction's own lvalue is always retained, so post-hook values still scope together; the cross-hook value becomes a scope dependency instead.
• MergeOverlappingReactiveScopesHIR.visitPlace — don't re-merge inner scopes into a hook-spanning outer scope for non-mutating references (!isMutableEffect). A plain read/freeze does not require the merge for correctness; only a real mutation does, so reads stay independently memoizeable.

Result: the ref callback memoizes independently (depending on the stable ref setter); an expression that genuinely captures the value still merges and stays un-memoized, which is correct.

Test plan

• New fixture ref-callback-with-hook-spanning-dependency — ref callback is now memoized; sprout eval passes.
• Full fixture suite: no regressions. One fixture (todo-round3_promote_used_temps) gains memoization (cache size grows); its expect is updated.
• typecheck + eslint clean.