5.7.0 - Coloured teletext + page override

Coloured teletext + selectable caption page (#107)

Added

• Coloured DVB teletext captions. Teletext now decodes through libzvbi as ASS, so the per-character colour broadcasters use to distinguish speakers reaches the overlay. Coloured pages publish as the new SubtitleCue.Body.richText([SubtitleTextRun]) (each run an optional RGB SubtitleColor, nil meaning the host default); cue.text still flattens rich cues to plain text, and an all-white page keeps emitting plain .text, so existing text consumers are unchanged. Thanks to @tresby and @nathanpiper.
• Teletext caption-page override. LoadOptions.teletextPage selects the decoded teletext page (default: libzvbi auto-detect of the flagged subtitle page). Channels whose captions ride a page libzvbi does not flag (for example Australian free-to-air on page 801) can now be targeted explicitly; the option threads through every subtitle tap site.

Fixed

• Coloured teletext cues are trimmed and de-duplicated in the retained store like plain ones: a coloured caption is closed by its successor instead of lingering to the page-hold cap, and does not duplicate across a live-DVR seek.

No FFmpeg rebuild; FFmpegBuild 2.0.0 is unchanged. Full notes in the CHANGELOG.