Implements item 21 from issue #150.

Summary

The four text-heavy rules — pii-redact, secrets-redact, encoded-payload-redact, prompt-injection-redact — used to scan large innerText synchronously, blocking the event loop for hundreds of ms on dense pages and producing a cross-tree mutation hazard when an SPA route swap landed mid-scan. This PR:

1. Introduces lib/yielding-text-walk.ts — a chunked TreeWalker over text nodes that yields between chunks (default 100 nodes) and accepts an optional AbortSignal. Yield mechanism is requestIdleCallback (yields to layout/paint) with a setTimeout(0) fallback.
2. Wires each of the four rules to a ReusableAbortController from abort-utils. Route-change events (via the existing subscribeRouteChange) call abortAndReset on each rule independently, so an in-flight scan against the old tree stops cleanly when the SPA swaps to the new one.

Incremental subtree-watcher batches do NOT abort — they target their own scoped root and don't conflict with whatever else is running. The structural-reset signal is the only cancellation channel, as discussed in #159's risk audit.

Design choices

• Sync fast path: when the whole tree fits in one chunk (the common case for small subtrees + the default chunkSize=100), process and onComplete fire before the helper returns. Existing rule tests keep working without explicit await machinery — only the multi-chunk path (large pages, the abort tests) needs to flush microtasks.
• onComplete callback instead of returning Promise<void>: prompt-injection-redact collects containers in pass 1 then dedupes + hides in pass 2. The second pass must see the full container set, so it has to run after the last chunk — onComplete handles that uniformly whether the walk was sync or chunked.
• Resume node pre-fetch: process(chunk) detaches the chunk's text nodes (the consumer rules call replaceMatchesInTextNode), so the walker's currentNode becomes a detached node and nextNode() returns null. The fix pre-fetches the next node before process and re-anchors walker.currentNode to it before yielding. Caught by review feedback on the initial commit; regression tests added.
• abort-utils CJS stub in src/__test-mocks__/abort-utils.ts, wired via moduleNameMapper. The package is ESM-only and ts-jest's CJS pipeline can't transform it; the stub mirrors the runtime semantics each rule depends on (signal flips on abortAndReset, onAbort dispatch). Three pre-existing test files still carry inline jest.mock("abort-utils", …) blocks that take precedence over the mapper — left in place to keep this PR focused.

Rebase notes

Rebased onto 4a10459 after #160 and #161 landed.

• Fix: prompt-injection-redact would re-hide a revealed container #161 (Fix: prompt-injection-redact would re-hide a revealed container) touched the same hide loop this PR refactors. Reconciled the closest(REVEALED_SELECTOR) skip check from Fix: prompt-injection-redact would re-hide a revealed container #161 by moving it into the onComplete callback alongside the existing isInsidePlaceholder / isConnected checks — same semantic guard, just at the new location after the chunked-walk refactor. The 2 new tests Fix: prompt-injection-redact would re-hide a revealed container #161 added (reveal flow describe block) auto-merged into the test file and still pass.
• Fix: disguised-ad-flag re-hid the article a user just revealed #160 (Fix: disguised-ad-flag re-hid the article a user just revealed) — no file overlap with this PR.

Test plan

• bun run test — 1285 / 1285 pass (4 new pass over pre-rebase baseline come from Fix: prompt-injection-redact would re-hide a revealed container #161's auto-merged tests)
• bun run typecheck
• bun run check (biome + eslint)
• bun run knip
• Helper unit tests (yielding-text-walk.test.ts): sync fast path, chunked path with microtask yield, signal handling (pre-aborted / mid-walk abort / mid-process abort), filter parity with walkTextNodes (NON_CONTENT_TAGS, placeholder skip, minLength, shouldSkipParent), and the resume-anchor regression where process detaches each text node.
• Helper property tests (yielding-text-walk.property.test.ts): for any tree + any chunkSize, the flat-mapped concatenation of all chunks equals walkTextNodes's output in document order — covers sync large-chunk and async small-chunk paths plus the chunk-size invariant.
• Per-rule abort tests: each of the 4 rules has a "teardown aborts the in-flight chunked walk" case. For prompt-injection-redact the assertion is stronger — onComplete is the hide pass, so an abort before it runs means no placeholders ever installed.
• Per-rule cross-chunk test: pii-redact builds 200 SSNs, drains timers, asserts all 200 get masked (catches the resume-anchor regression end-to-end).
• Reveal-flow tests from Fix: prompt-injection-redact would re-hide a revealed container #161: re-applying after a manual reveal must not re-hide the container; sibling containers with their own injection text must still get hidden.

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