feat: coordinate slate playout with interstitial ad breaks

[birme]

Summary

• Implements feat: coordinate slate playout with interstitial ad breaks #369: build the ad-break filler slate so it lines up exactly with the feat: emit HLS interstitial (EXT-X-DATERANGE) tags for configured ad breaks #368 interstitial EXT-X-DATERANGE. Builds on feat: add ad-break/interstitial channel configuration surface #367 (config) + feat: emit HLS interstitial (EXT-X-DATERANGE) tags for configured ad breaks #368 (emission).

Changes

• engine/session.js: add _buildAdBreakSlate(afterVod, unixTs, injectSlateLoaders). Uses HLSRepeatVod(slate.uri, slate.repetitions) so the slate run spans exactly repetitions * slate.duration / 1000s — the identical value _addInterstitialMetadata (feat: emit HLS interstitial (EXT-X-DATERANGE) tags for configured ad breaks #368) computes for PLANNED-DURATION. Attaches the interstitial DATERANGE to the slate VOD's first segment so START-DATE lands on the break boundary and the tag brackets exactly the slate run.
• Strips the single leading EXT-X-DISCONTINUITY that HLSRepeatVod prepends, so the DATERANGE lands on the true first segment (no vodlib change / no feat: emit HLS interstitial (EXT-X-DATERANGE) tags for configured ad breaks #368 rework).
• Gated on adBreak.enabled + a configured adBreak.slate.uri; returns null otherwise. The existing error/gap filler slate paths (_insertSlate/_loadSlate/_truncateSlate, slateUri/slateRepetitions/slateDuration) are untouched.

Integration note for reviewer

There is no ad-break scheduling/trigger mechanism anywhere in this series yet (the #367→#371 breakdown does not define one; #368 emits the tag at content-VOD boundaries). Accordingly this PR delivers the slate-building + exact-alignment logic and its spec coverage, but the helper is not yet invoked from a live trigger. Please assess whether that satisfies #369's acceptance given the series state, or whether it should block.

Test plan

• PROOF: npm run build → clean (no errors)
• PROOF: npm test → 100 specs, 0 failures, 7 pending specs (baseline 94; +6 specs asserting slate spans the window, DATERANGE brackets exactly the slate (single tag, correct START-DATE/PLANNED-DURATION), alignment holds at a different size, disabled/no-slate → null, non-ad-break slate untouched)

Closes #369

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

[birme]

pr-reviewer verdict: APPROVE (with a documented limitation + follow-up recommendation)

• PROOF: npm run build → clean; npm test → 100 specs, 0 failures, 7 pending specs.
• Slate run duration equals #368's PLANNED-DURATION by construction; DATERANGE attached to the slate's first segment so it brackets exactly the slate (single tag, correct START-DATE) — proven by spec.
• Strictly gated on adBreak.enabled + configured slate; existing error/gap filler slate paths untouched (spec-asserted). No commercial names/trademarks; operator-controlled URI (low risk); commit message conforms.

Judgment call (uncalled-in-production helper): investigated all production seams — _insertSlate is an error/gap-recovery path (wrong place), and content-VOD boundaries would force slate on every transition (wrong). AdBreakOpts carries no timing/cue field, so no legitimate trigger seam exists yet. This mirrors how #368 shipped emission with scheduling deferred. Approving as correct foundation code; recommend a follow-up issue to define the ad-break trigger and wire _buildAdBreakSlate into it.

Awareness note: alignment relies on stripping HLSRepeatVod's leading EXT-X-DISCONTINUITY via regex — coupled to vodlib output format, but guarded by the "brackets exactly the slate" spec.