Release v6.4.0 — adopt engine onIter, fix arrivalPath ordering bug

[mellonis]

Summary

Adopts `@turing-machine-js/machine` v6.4.0's new `onIter` hook to fix a pre-existing `arrivalPath` ordering bug that's been latent since v6.1.0. Also surfaces `onIter` as a new `pm.run()` parameter forwarded to the engine.

Version skips 6.2.0 and 6.3.0 — both were prepared but neither shipped to npm (see CHANGELOG history note).

The bug

Since v6.1.0 the internal `onStep` wrapper advanced `prev` mid-iter, which races engine v6.0.0+'s per-iter `before → step → after` dispatch order on the same yield. By the time `onPause(after, K)` fired:

• `prev` had already advanced to iter K's own state (from `onStep`'s wrapper)
• `m.arrivalPath` resolved to K's next instruction, not K's own
• The registry-aware `#shouldFireOnPause` filter then saw the wrong path and silently dropped user-registered `{ after: true }` breakpoints — they were undocumentedly invisible.

Regression test added in `test/breakpoints.spec.ts` — fails on master, passes after the fix.

The fix

Move `advanceTracking` from the internal `onStep` wrapper to a new internal `onIter` wrapper. `onIter` fires at end-of-iter — after both `onPause` dispatches on the same yield have already read their iter-correct `prev` — so the advance no longer races them. Required the new `onIter` engine hook (turing-machine-js v6.4.0).

Also added

• `onIter` parameter on `pm.run()`: `onIter?: (m: MachineState) => void | Promise`. Forwards to the engine's `onIter` with PostMachine's wrapped `MachineState` (so `m.arrivalPath` and `m.candidatePaths` are populated). Use for per-iter coordination — throttle, animation, yield-to-other-work.

Three-hook contract recap (matches engine v6.4.0)

Hook	Sync/Async	When	Use
`onStep`	Sync, not awaited	Mid-iter	Cheap tracer/logger
`onPause`	Awaited	On registered breakpoint match (arrival-aware filter)	User breakpoints / debugger UI
`onIter` (new)	Awaited	End of every iter	Per-iter coordination

Compatibility

• Engine peer-dep: `^6.0.0` → `^6.4.0`. Required because v6.4.0 added the `onIter` hook the fix depends on. Consumers on engine v6.0.x – v6.3.x must upgrade the engine alongside this package.
• Source-level API: purely additive (new `onIter` parameter). No API breaks for consumers that don't opt in.
• Internal `onStep` wrapper is now conditional (registered only when user provides `onStep`), since it no longer carries the always-on `advanceTracking` side-effect. Engine zero-cost when the consumer provides no callbacks at all.

Release plan after merge

• `cd packages/machine && npm publish`
• `gh release create v6.4.0 --target master --title v6.4.0 --notes-file ` (stable, not pre-release per the established pattern)

Test plan

• `npm test` — 266/266 against published engine v6.4.0 (was 265, +1 from the new regression test)
• `npm run typecheck` — clean
• `npm run lint` — clean
• `npm run build` — clean