feat: stopWhen / stopWhenSync source adapters (0.9.2)

[ildella]

What

New source adapter, promoted from nucube-app where it already replaced hand-rolled stop logic in the MusicBrainz enrichment pipeline (second time the pattern was requested by a consumer):

```js
export const stopWhen = (items, predicate = () => false) => ({
async * [Symbol.asyncIterator] () {
for await (const item of items) {
if (predicate(item))
return
yield item
}
},
})
// + stopWhenSync with Symbol.iterator
```

Usage — cancellation lives in the source, everything else stays in its own place:

```js
await series(enrichOne, {
total: albums.length,
pause: ENRICH_DELAY_MS,
pauseOnErrors: true,
onProgress: onItem,
})(stopWhen(albums, shouldStop))

stopWhen(folders, () => count >= limit) // limits close over counters
```

Contract (pinned by tests)

• Check before yield — the triggering item is pulled but never processed ("cancel before work")
• Pull-lazy — once stopped, the underlying source is abandoned mid-stream; native cleanup (`finally`, `iterator.return()`) still runs (locked with `trackedSource`)
• Clean completion, not an error — consumers see `failure: false`, empty `sourceErrors`; cancel is a shorter run
• Predicate throws are source errors — `onSourceError` / `sourceErrors`, never `onError`
• Predicate is sync, receives `(item, index)`, defaults to `() => false` (identity wrapper)
• Length forwarding — array sources forward `.length` on the wrapper so `series` keeps progress totals without an explicit `total`; generators keep total omitted
• Composes with `series` / `scan` / `reduce` / `filter` / `findSync` and raw `for await` with zero changes to those functions

Design notes for reviewers

• Source wrapper, not a `series` option. An option would only cover `series`/`filter`; it would never reach `reduce(ingestEvents())` or raw `for await` loops. The wrapper helps every consumer at once. A native `while:` option was considered and rejected; if we ever want one, `stopWhen` still stands alone.
• Not intra-item abort. Work already started on an item is not interrupted — that stays in the operation (AbortController). Documented in reference/patterns.
• Bug fix included: `filter` / `filterSync` / `findSync` sniff their first argument and treated any non-array object as a `where()` pattern — so `filter(pages(), pred)` silently returned a curried function instead of running. `isPatternObject` (in `shared.js`) now excludes objects implementing an iteration protocol. Patterns (`{active: true}`) are unaffected; regression tests cover generators directly.

Out of scope (explicitly)

• native `while`/`until` option on consumers · async predicates · currying · `takeWhile` alias · changes to `take`

Testing

34 new tests (`tests/stop-when.test.js`, `tests/stop-when-sync.test.js`): nucube's 7 ported verbatim in intent + composition (series/reduce/filter/take), length forwarding, index passing, closure predicates, predicate-throws-as-source-error, generator cleanup. Full suite: 400 passed. Lint clean.

Docs

reference.md (new entry + TOC), guide.md, patterns.md (Pattern 9), examples.md (MusicBrainz-shaped example), architecture.md (one sentence under Eager Execution), migration.md (0.9.2 section incl. the filter fix note), skills/core/SKILL.md, README. Version bumped to 0.9.2.

Follow-up (not here)

nucube-app can delete `src/lib/iterables/stop-when.js` and import from `pipelean@0.9.2` with no call-site change; it may then drop its explicit `total: albums.length`.