fix(catalog): assign external subtitle track indexes

[RXWatcher]

Summary

• assign each external subtitle its ordinal in the playback combined-index space
• preserve existing ffprobe indexes for embedded subtitle streams
• add regression coverage for multiple external subtitles alongside embedded tracks

Without this, every external subtitle serialized with index 0, producing duplicate client keys and allowing the wrong subtitle track to be selected.

Verification

• go test ./internal/catalog
• git diff --check upstream/main...HEAD

Summary by CodeRabbit

• Bug Fixes

  • Fixed external subtitle tracks receiving duplicate or ambiguous indexes.
  • Improved subtitle selection and URL resolution when media includes both embedded and external subtitles.

• Tests

  • Added coverage verifying unique indexes and correct external-track identification.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: e60d146b-edf4-4e73-8869-a184d07c776b

📥 Commits

Reviewing files that changed from the base of the PR and between 63be693 and 4825d3d.

📒 Files selected for processing (4)

• internal/catalog/detail.go
• internal/catalog/detail_subtitle_tracks_test.go
• internal/jellycompat/streams.go
• internal/jellycompat/subtitle_selection_test.go

📝 Walkthrough

Walkthrough

External subtitle tracks now receive deterministic sequential indexes during track construction. A new test verifies external index uniqueness, embedded index preservation, track count, and external-track marking.

Changes

Subtitle Track Indexing

Layer / File(s)	Summary
External index assignment and validation internal/catalog/detail.go, internal/catalog/detail_subtitle_tracks_test.go	buildVersionSubtitleTracks assigns sequential indexes to external subtitles, while the new test verifies combined track identity and preserved embedded indexes.

Estimated code review effort: 2 (Simple) | ~5 minutes

Suggested labels: v1

Suggested reviewers: quick104

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: assigning indexes to external subtitle tracks in catalog.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@internal/catalog/detail.go`:
- Around line 3415-3421: The external subtitle loop in
internal/catalog/detail.go lines 3415-3421 must assign collision-free full
playback indexes, not local ordinals: follow the same positive-index and
zero-fallback contract used by downstream selectors, accounting for video,
audio, and embedded subtitle indexes. Add collision cases and assert both
consumer-facing and serialized indexes in
internal/catalog/detail_subtitle_tracks_test.go lines 15-42.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: f392177d-70fc-4d98-8437-81ae4c686a97

📥 Commits

Reviewing files that changed from the base of the PR and between 0a91444 and 63be693.

📒 Files selected for processing (2)

• internal/catalog/detail.go
• internal/catalog/detail_subtitle_tracks_test.go