[rust-compiler] Dedup async-reassignment immutability diagnostic

[poteto]

Bug

validate_locals_not_reassigned_after_render emitted the same
Cannot reassign variable in async function diagnostic once per
async function body that reassigned the same outer let. The TS
reference emits it exactly once per offending variable.

Reproducer (both compilers, panicThreshold: 'none'):

function Component() {
	let value: number | undefined;
	const a = async () => { value = 1; };
	const b = async () => { value = 2; };
	return <div>{[a, b]}</div>;
}

Before: TS emits 1 error, Rust emits 2.
After: both emit 1.

Root cause

crates/react_compiler_validation/src/validate_locals_not_reassigned_after_render.rs is a faithful port of the TS pass, except the inner-function async-reassignment branch is missing the early return. The TS reference does env.recordError(...); return null;. The Rust code recorded the diagnostic then fell through into the next loop iteration, so the next async function body got the same emit path.

There was even a // Return null (don't propagate further) — matches TS behavior comment in place of the missing return.

Fix

Replace the comment with the actual return None;. One-line change.

How I found it

TS-vs-Rust differential sweep over a large real-world TSX corpus (~59k files), comparing the in-repo TS compiler vs the Rust port. Among 14k+ error events, this duplicate emission was the most concentrated divergence cluster.

Verification

• New fixture error.invalid-reassign-local-variable-in-multiple-async-callbacks.tsx: yarn snap and yarn snap --rust both pass (1 error each).
• The existing error.invalid-reassign-local-variable-in-async-callback.js fixture still passes.
• Full yarn snap --rust: 1782/1794 (+1 vs main, no regression in the 12 known frontier failures).

Commit order

1. Adds the fixture with the broken-Rust baseline (Found 2 errors). yarn snap --rust passes against it, yarn snap fails — captures the bug.
2. Applies the one-line fix and flips the baseline to Found 1 error. Both yarn snap and yarn snap --rust pass.