fix(index): yield during resolution so the liveness watchdog can't kill a valid large index (#1091)

[colbymchenry]

Problem (#1091)

Indexing a large project gets SIGKILLed partway through with "Main thread unresponsive for ~60s — killing the wedged process (#850)", even though the index is healthy and progressing. Users were forced to set CODEGRAPH_NO_WATCHDOG=1. Reported on both a 22k-file repo (bar at 100% then wedge) and a 134-file repo (88%).

Root cause

The #850 liveness watchdog SIGKILLs a process whose main-thread event loop stalls past its window (60s default). Its heartbeat is a setInterval on that thread. It was wired into index/init in #999 — but unlike the serve daemon (whose heavy work is off-thread in the parse worker / shelled out), reference resolution and callback-edge synthesis run synchronously on that same thread. On a large repo those spans legitimately run for minutes, so the heartbeat can't fire and the watchdog kills valid, in-progress work. The progress bar freezes wherever it last rendered (88% / 100%) when a synchronous span begins.

Fix

Make the long synchronous spans yield cooperatively (src/resolution/cooperative-yield.ts, a budget-gated maybeYield()), so the heartbeat keeps firing during real work while a genuinely wedged span — which never reaches a yield — still trips the watchdog:

• synthesizeCallbackEdges yields between its ~33 whole-graph passes, and the heavy scanners (closure-collection, event-emitter, JSX-child, object-registry, field-channel) yield within their loops;
• batched resolution sub-chunks each batch with yields;
• the deferred chained-call and this-member post-passes yield per ref.

Behaviour-preserving — only timing changes; node/edge counts are identical.

Validation (real watchdog armed at the default 60s)

Test	main	this branch
Swift compiler (27k files, 441s resolving, ~1.1M edges)	KILLED @60s (dies at 644s, in the end-stage post-passes)	complete graph — 0 unresolved refs, 99.99% of baseline edges, no kill (×3 runs)
Swift C++ subset (4.9k files, 165s resolving)	completed	COMPLETED @60s (node/edge parity, 1-edge nondeterminism)
TypeScript compiler	KILLED @1–2s	COMPLETED

Full test suite green (1890). New regression test __tests__/cooperative-yield.test.ts pins the yielder's budget contract and locks the async structure of the three spans.

Closes #1091.

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