feat: emit HLS interstitial EXT-X-DATERANGE tags for configured ad breaks

[birme]

Summary

• Implements feat: emit HLS interstitial (EXT-X-DATERANGE) tags for configured ad breaks #368: emit HLS-interstitial EXT-X-DATERANGE tags for a configured ad break (builds on the feat: add ad-break/interstitial channel configuration surface #367 config surface). Asset resolution via the ad-serving endpoint is feat: integrate configurable ad-serving endpoint for interstitial asset resolution #370; slate/duration alignment is feat: coordinate slate playout with interstitial ad breaks #369.

Changes

• engine/session.js: add _addInterstitialMetadata(vod, timestamp), invoked at the two content-VOD creation sites (after the existing timedMetadata loop). Reuses the existing per-VOD addMetadata/rangeMetadata daterange pathway — the same mechanism slate VODs already use — so no @eyevinn/hls-vodtolive change is needed (verified the vodlib renders arbitrary daterange keys).
• Emits #EXT-X-DATERANGE:CLASS="com.apple.hls.interstitial",ID=...,START-DATE=...,PLANNED-DURATION=...,X-ASSET-URI=.... CLASS is the Apple HLS-interstitial spec identifier. Asset source: configured slate.uri, falling back to adServerUri. PLANNED-DURATION derived from slate repetitions×duration (omitted when no slate duration).
• Strictly gated on adBreak.enabled: when disabled (the default) the helper is a no-op and manifests are byte-identical to before.

Test plan

• PROOF: npm run build → clean (no errors)
• PROOF: npm test → 94 specs, 0 failures, 7 pending specs (baseline 90; +4 specs: enabled w/ slate attributes, enabled fallback to adServerUri, disabled-vs-enabled byte-identical after stripping injected lines, no-op when disabled)

Closes #368

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

[birme]

pr-reviewer verdict: APPROVE

• PROOF: npm run build → clean; npm test → 94 specs, 0 failures, 7 pending specs.
• Emits EXT-X-DATERANGE with CLASS="com.apple.hls.interstitial", ID/START-DATE/PLANNED-DURATION/X-ASSET-URI via the existing addMetadata/rangeMetadata daterange pathway (no vodlib change). com.apple.hls.interstitial is the required Apple spec identifier, not a trademark issue.
• Gate verified correct: disabled (default) = no-op; the disabled-vs-enabled spec proves byte-identity after stripping injected lines.
• Commit message conforms.

Notes (non-blocking, follow-ups):

• The vodlib's daterangeAttribute doesn't escape attribute values; X-ASSET-URI is operator-supplied config (not end-user), so low risk — worth upstream URI validation later.
• Emission fires at every content-VOD boundary since no break-scheduling exists yet; break/slate alignment is deferred to #369.