chore: migrate engine utility and support modules to TypeScript

[birme]

Summary

• Implements chore: migrate engine utility and support modules to TypeScript #375: convert the remaining helper modules (util.js, preflight.js, subtitle_slicer.js, chaos_monkey.js) to TypeScript — type-only, no behavioral change. Depends on chore: establish incremental TS migration scaffolding and CI gate #372 (scaffold, merged).

Changes

• git mv .js→.ts for all four; module.exports shapes preserved exactly (util: named-export object of 17 helpers; preflight: module.exports.handler; subtitle_slicer: class default; chaos_monkey: { loadVod }) so all consumers (session*.ts, state stores, server.ts, specs) resolve unchanged.
• Concrete types for pure helpers (timer, findNearestValue, isValidUrl, m3u8Header, loadVod, …); any at dynamic interop (m3u8 objects, node-fetch/AbortController, restify req/res, Buffer/stream plumbing).
• Class fields declared with declare (WaitTimeGenerator, SubtitleSlicer) so no field initializer is emitted under target es2022's default useDefineForClassFields — constructor assignments stay the sole runtime writes. Neither class uses extends.
• Pre-existing latent bug in subtitle_slicer (this.vttFile[uri] guarded by never-true this.vttFiles.length) preserved as-is (not fixed) to avoid behavior change.

Test plan

• PROOF: npm run build → tsc clean; npm test → 113 specs, 0 failures, 7 pending; NODE_OPTIONS=--no-experimental-strip-types npm test (CI Node 20 path) → 113/0. Diff is annotation-only under --ignore-all-space.

Closes #375

🤖 Automated via Channel Engine Dev daily-backlog-pr skill

[birme]

pr-reviewer verdict: APPROVE

PROOF: npm run build clean; npm test → 113 specs, 0 failures, 7 pending; NODE_OPTIONS=--no-experimental-strip-types npm test → 113/0. CI green on 20.x/22.x.

Faithful type-only port. All four export shapes byte-identical to master. --ignore-all-space diff shows only erasable syntax (annotations, ? markers, as any/as number casts). Spot-checked fetchWithRetry, filterQueryParser/applyFilter, codecsFromString, WaitTimeGenerator, findNearestValue — no conditionals/bounds/defaults altered. Verified in emitted dist/ that declare fields (WaitTimeGenerator, SubtitleSlicer) produce NO initializers, so the es2022 useDefineForClassFields clobber is avoided and constructor assignments remain the sole writes. The three no-op casts are runtime-identical to the original coercions.

Non-blocking: pre-existing latent dead-code bug in subtitle_slicer (this.vttFile[uri] guarded by never-true this.vttFiles.length) confirmed identical on master and correctly preserved — recommend a follow-up bug issue. Merging.