fix(multiple-subtitles): repair old ASS conversion line breaks [PKG-MULTI-SUB-11]

refactor/changes/2026-09-14-PKG-MULTI-SUB-11-ass-conversion.md

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+# PKG-MULTI-SUB-11: preserve ASS conversion on old core main
+
+## Reproduction
+
+Frozen npm core5.1.2 has different converter behavior across its own files.
+For a plain Dialogue cue, main returns
+`WEBVTT 1 00:00:01.250 --> 00:00:03.750 Plain`; the source and legacy file retain
+the required header, identifier, timing and payload line breaks. The plugin's
+VTT parser consequently sees no ASS cue from main. These are actual verified
+tarball members, not simulated converters or version-number assumptions.
+
+The new unit regression initially passes seven baseline/legacy assertions and
+fails six main-converter samples. The initial raw-string text assertion also
+compared an encoded ampersand with an equivalent literal ampersand; that fixture
+failure is retained separately. Canonical HTML comparison fixes the test's text
+representation comparison without weakening cue timing/count or semantic markup.
+The actual browser regression with the previous plugin artifact fails because
+only two of three VTT/SRT/ASS cues load on5.1.2.
+
+## Implementation and compatibility
+
+`src/ass-conversion.ts` is a private adapter called after request decoding. It
+calls the existing host converter once. Valid output takes the normal path.
+For output with the old collapsed header/timing shape, the adapter reconstructs
+the old converter's supported Dialogue fields, timestamp formatting and text
+cleanup, then compares the entire collapsed output. Only an exact match receives
+the missing structural newlines. Custom output, comments, empty results and
+nonmatching malformed conversions retain their original value; exceptions still
+reject registration and release owned resources.
+
+This small duplicate of the old supported Dialogue projection is intentional:
+importing runtime code from the new core would make the plugin depend on that
+core version. It is not a second general ASS parser. Keep its complete-output
+comparison and frozen source/legacy parity tests when editing it. Do not extend
+accepted ASS syntax or modify cue timing under this repair task.
+
+The host's shared utility is never patched, called with a probe or replaced.
+The factory, methods, event order, public types, declarations, global/ESM/CJS
+entries, vendor bytes and package versions stay unchanged. No dependency or
+lockfile changes are needed. The new test joins existing Yarn unit/package
+commands; all three package formats and docs copies are normally rebuilt.
+
+## Validation and limits
+
+Exact inputs, hashes, archived reports and commands are in
+[the validation index](../baselines/multiple-subtitles-ass-validation.json).
+
+- Plugin suite315/315: the new15 tests include actual old main/legacy converters,
+  source parity, independent timing, Unicode, inline markup, ASS line breaks,
+  overlapping cues, long hours, comma text, custom/empty output and failure cleanup.
+- The same targeted15-test file passes with actual main and legacy plugin input.
+  Its frozen-host and private-adapter checks are identified separately from
+  full-plugin registration/selection cases in the source.
+- Source browser21/21 and actual-main browser21/21. The matrix spans
+  core5.1.2/5.1.7/5.3.0/5.4.0/candidate in Chromium, Firefox and Windows WebKit.
+  It checks UTF16 input, explicit type override, native cue times/text, multiline
+  and semantic HTML, overlapping translation, selection/reset and clearing.
+  The old-host regression now uses an actual third ASS track on both5.1 cores.
+- Actual legacy browser21/21. Strict TypeScript,37 declarations with zero drift,
+  root and changed-script lint, CI50 and plan/risk validation pass; root lint keeps
+  the existing generated-doc warning. Detailed evidence is retained in the index.
+
+Close only MULTI-SUB-ASS-01 after these checks. This is conversion into VTT, not
+full ASS positioning/animation/rendering, physical-device evidence, remote CI or
+package publication. The old WebKit source-seek failure reopened in task10 stays
+open and is not rerun or waived by this targeted conversion matrix. Tasks05/06
+and the wider refactor remain incomplete.
+
+## Maintenance and rollback
+
+The package architecture and test READMEs describe the adapter and replay commands.
+Revert this task and rebuild the plugin to restore prior behavior; preserve the
+frozen converter evidence so the known old-main failure cannot be mistaken for
+successful ASS compatibility. No push or publication is included.