fix(multiple-subtitles): render overlapping cues on legacy hosts [PKG-MULTI-SUB-10]

refactor/changes/2026-09-14-PKG-MULTI-SUB-10-legacy-captions.md

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+# PKG-MULTI-SUB-10: old-core simultaneous caption rendering
+
+## Problem and boundary
+
+Actual core5.1.2/5.1.7 render only `subtitle.activeCue`, even when the native track
+contains multiple active cues. Plugin1.0.0 combined tracks by array index;
+1.1.0/1.2.0 and the prior candidate preserve independent timing and consequently
+lose visible languages on these hosts. The corrected regression fails on both
+old cores with the previous candidate main artifact, at the missing second caption.
+
+The first regression also exposed a separate5.1.2 ASS conversion defect: its
+published main bundle collapses required VTT line breaks into spaces, whereas its
+tarball source preserves them. This yields two native cues rather than three.
+Task11 owns that fix. The rendering regression now uses VTT/SRT on both cores and
+additionally ASS on5.1.7; substituting the third track does not constitute ASS
+acceptance on5.1.2. Original failed runs remain in the evidence index.
+
+## Implementation and compatibility
+
+`caption.ts` detects the old `activeCue` surface without the newer `activeCues`
+surface, using property existence without invoking getters. It registers one old
+`subtitleUpdate` listener instead of the modern `subtitleAfterUpdate` listener.
+`legacy-caption.ts` renders all native active cue texts in their original order
+when multiple cues are active. Single/empty intervals retain core rendering.
+The existing timestamp cleanup runs after either core path.
+
+No cue timing, native object, host method identity, source clock or event payload
+is replaced. The old scalar event still contains only the first cue text. Existing
+listeners keep their registration order; those before the plugin see the original
+first-cue view, while later listeners see the combined view. Applications making
+custom caption DOM replacements should register that listener after the plugin.
+The plugin already owns this caption layer and sets escape=false; if the user
+later enables escape, the adapter preserves literal text and old per-line classes.
+
+The lifetime owns listener removal and rejects late callbacks after destroy.
+The adapter has no timer, media request or new runtime dependency. Public types,
+factory methods, export aliases, package versions and vendor code are unchanged.
+The nearby serializer receives only the braces required by its existing lint rule.
+
+## Verification
+
+Exact hashes, archived reports and browser versions are in
+[the validation index](../baselines/multiple-subtitles-legacy-validation.json).
+
+- Six new Node regressions cover native order, repeated updates, scalar payloads,
+  listener order, original method identity, single/empty intervals, escape/CRLF,
+  timestamp cleanup, modern isolation and destroy/registration failure cleanup.
+- The complete plugin suite passes300 tests, including the historical runtime and
+  public type suites. The final listener-order assertions also pass against actual
+  main and legacy artifacts, six cases each.
+- Source browser matrix:105 cases,104 pass and one old5.4.0/WebKit source-seek
+  failure. All six new old-host overlap cases pass. Historical missing languages
+  and paused-offset observations remain labelled, not counted as correct behavior.
+- Actual main matrix:63 cases,62 pass and the same old5.4.0/WebKit failure.
+  Its six old-host overlap cases pass; native cue identities, independent time
+  windows, semantic HTML/CSS, repeated updates, selection/reset and clearing hold.
+- Actual legacy matrix passes12/12: six old-host cases and six modern entity cases.
+- Strict package TypeScript, root lint and explicit changed-test lint pass; root
+  lint retains one existing generated-doc warning. CI regression suite passes50.
+  The normal build regenerates all three package formats and matching docs copies.
+
+The broad matrix reopens MULTI-SUB-SWITCH-01: waiting for seeking=false did not
+make every old-host subsequent seek reliable. The source failure retains its
+1..3-second cue but media time is0.001494 with no active cues. Task05 owns renewed
+diagnosis; this adapter does not run on that modern host. No retry, skip, timeout
+increase or passing rerun is used to erase those failures. The complete package,
+physical-device matrix, distribution gate and full refactor are not finished.
+
+## Maintenance and rollback
+
+Use the legacy browser spec for old-host display changes and combinations for
+historical profile comparisons. Keep converter, native seek and caption display
+failures separate. The package architecture and test READMEs map these entrypoints.
+Revert this task's commit and rebuild the package to remove the adapter; retain
+the failures and task11 in the working plan. No pushing or publishing is included.