chore: document HLS-interstitial ad-break configuration and behavior

[birme]

Summary

• Implements chore: document SGAI HLS-interstitial ad-break configuration and behavior #371: document the HLS-interstitial ad-break feature now that its implementation sub-issues (feat: add ad-break/interstitial channel configuration surface #367 config surface, feat: emit HLS interstitial (EXT-X-DATERANGE) tags for configured ad breaks #368 EXT-X-DATERANGE emission, feat: coordinate slate playout with interstitial ad breaks #369 slate coordination, feat: integrate configurable ad-serving endpoint for interstitial asset resolution #370 ad-endpoint resolution) have all landed on master.

Changes

• docs/reference.md: new "Ad-break configuration (HLS interstitials)" section — field tables for adBreak (enabled, adServerUri, slate.{uri,repetitions,duration}) with types/defaults/validation, plus the ad-replacement flow.
• README.md: Features bullet, an "Ad breaks (HLS interstitials)" subsection linking to the reference, and the new example in the examples list.
• examples/adbreak.ts: minimal channel wired with a per-channel adBreak (enabled + adServerUri + slate) and an engine-level disabled default, matching default.ts style. Vendor-neutral throughout (uses ad-endpoint.example.com).

Test plan

• PROOF: npm run build && npm test → 113 specs, 0 failures, 7 pending. New examples/adbreak.ts compiles under the repo tsconfig (part of the TS build).

Closes #371

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

[birme]

pr-reviewer verdict: APPROVE

PROOF: npm run build && npm test → 113 specs, 0 failures, 7 pending.

Accuracy cross-checked against code (all match):

• adBreak.enabled (bool, default false), adServerUri (required-when-enabled, validated absolute http/https) — server.ts AdBreakOpts/normalizeAdBreak.
• slate.{uri, repetitions=10, duration=4000} — server.ts SlateOpts + defaults.
• 2s timeout, Accept: application/json, {assetUri}|{assetList}|{assets:[]} → X-ASSET-URI/X-ASSET-LIST, EXT-X-DATERANGE com.apple.hls.interstitial emission, PLANNED-DURATION = reps×duration/1000, slate fallback on timeout/non-2xx/malformed — session.js.

No trademarks (example uses ad-endpoint.example.com). examples/adbreak.ts compiles as part of the TS build. Style matches existing examples/docs.

Non-blocking note for a future code PR: engine/server.ts:34-35 carries a stale comment saying interstitial tags aren't emitted yet (they now are) — pre-existing on master, out of scope for this docs PR. Merging.