test(multiple-subtitles): [PKG-MULTI-SUB-05] checkpoint native core combination gaps

refactor/changes/2026-09-14-PKG-MULTI-SUB-05-combinations.md

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+# PKG-MULTI-SUB-05: core combinations checkpoint
+
+Baseline: 675b095b27e50832c4468096502e936422ea689a. Task05 remains doing.
+This checkpoint adds executable integration evidence, not a completed package or
+release acceptance. Source implementation, public types and generated bundles do
+not change. No dependencies, versions, lockfile, push or publication changes.
+
+## Inputs and tests
+
+`multiple-subtitles-cores.json` freezes real npm core5.1.2 metadata, SHA-512 archive
+integrity, SHA-256 archive/member inventory and manifest. It corresponds to the
+plugin1.0.0 historical gitHead association. Core5.1.7 is already frozen for VAST;
+5.3.0 is an adjacent stable comparison only. The actual registry request for5.3.1
+returns404, matching the earlier investigation; it is not silently substituted.
+Existing published5.4.0 and candidate core source complete the five-core set.
+These associations are not a declared minimum support range.
+
+The browser server verifies5.1.2 bytes before serving them. The new combination
+suite uses actual1.0.0/1.1.0/1.2.0 main bytes and the candidate source bundle.
+It verifies video playback/seeks, native VTT/SRT timing, track selection, offsets,
+web/native fullscreen, source change, multiple players and plugin-owned Blob URLs.
+The fixture uses native track.cues rather than the later subtitle.cues getter.
+Removal assertions use destroy(); destroy(false) intentionally retains HTML.
+Both mistakes were exposed and corrected in the initial test run; neither was a
+production defect. The original failed reports remain in the evidence manifest.
+
+## Actual findings and limits
+
+- Core5.1.2/5.1.7 only display activeCue, the first native active cue. Plugin1.0.0
+  combines languages into one cue and displays both. Plugin1.1.0/1.2.0/candidate
+  retain separately timed cues and display only the first language. All three
+  engines reproduce this exact distinction. Native tracks contain both languages;
+  selecting the second language alone still works. The24 boundary observations
+  include18 defect observations, not24 successful compatibility acceptances.
+  MULTI-SUB-MERGE-01 remains open for a compatible old-host solution.
+- Firefox155 at paused time1.5 still reports cues with start2/end4 as active after
+  subtitleOffset=1. HTML retains both captions. This occurs on5.3.0,5.4.0 and the
+  candidate, twice. The second run records native cues, active list, time, pause
+  state and rendered text. The candidate failure remains a real failing assertion,
+  not an expected-failure annotation, skip or timeout increase.
+- Windows WebKit26.6 on5.3.0 once lost the selected caption after source change;
+  the diagnostic rerun passed. This is an unresolved intermittent observation,
+  not proof of a fixed issue or a reproducible production root cause.
+- The full three-engine run:57 cases,53 passing assertions,4 failures,0 skips.
+  It includes the24 explicitly labelled boundary observations above. The targeted
+  diagnostic rerun:9 cases,6 pass,3 Firefox failures,0 skips. Earlier Chromium
+  runs had7 failures from fixture assumptions, then2 real overlap failures after
+  those assumptions were corrected. Current test adds final-state attachments;
+  the full run predates that diagnostic-only hook and formatting cleanup.
+
+Detailed input hashes, browser versions, reports, failure messages and selected
+native-state observations are stored in
+[multiple-subtitles-combinations-validation.json](../baselines/multiple-subtitles-combinations-validation.json).
+Local reports/traces are archived under refactor/.cache; their original attachment
+paths can be resolved relative to each named archive. The JSON retains small
+observations in Git so findings do not rely solely on those local archives.
+
+## Next actions and maintenance
+
+Fix the paused-offset candidate defect with an independent implementation task,
+first reducing it to a native-text-track probe and a controlled regression. Keep
+old release defects explicit. Resolve the old-core overlap behavior without
+changing legal public calls; investigate the intermittent old-WebKit switch loss.
+Then complete remaining ASS/encoding, overlap/offset transitions, lifecycle,
+core/device and distribution evidence. Do not mark05 done on this checkpoint.
+
+Run `yarn test:browser test/browser/multiple-subtitles-combinations.spec.js` to
+rerun the matrix. Its currently failing offset assertions intentionally prevent a
+green compatibility claim until corrected. The test supports the existing
+ARTPLAYER_MULTIPLE_SUBTITLES_ARTIFACT override for actual bundle verification;
+this checkpoint used source builds. No package rebuild is required for test/server
+changes. Revert this task05 checkpoint to remove the added tests and frozen core
+route; do not revert the independent task08 implementation commit with it.