OCTO-11513: Re-emit STYLE blocks and writing direction on VTT→VTT

[OlteanuRares]

Summary

• WebVTT writer now re-emits STYLE blocks when the CaptionSet originated from a VTT source (detected via ::cue marker key)
• Writer emits vertical:rl / vertical:lr from Layout.writing_direction when passthrough positioning isn't available
• Refactored monolithic pycaption/webvtt.py (1200 lines) into a pycaption/webvtt/ package (reader, writer, constants) — public API unchanged

Changes

• writer.py — new _build_style_block() emits global ::cue before class-specific rules; _convert_positioning() emits vertical setting

• reader.py — inserts ::cue marker in _parse_style_blocks() to signal VTT origin to the writer

• geometry.py — as_percentage_of() and fit_to_screen() now preserve writing_direction

• pycaption/webvtt.py → pycaption/webvtt/ package (__init__.py, constants.py, reader.py, writer.py)

Safety

• SCC/DFXP/SAMI inputs are unaffected — they produce no ::cue key, so _build_style_block() returns empty
• No duplicate positioning — passthrough short-circuits before Layout-based reconstruction
• 7 new tests cover: STYLE round-trip, global + class ordering, writing direction, passthrough dedup, regression (plain cue), cross-format filtering (SAMI, SCC)

Test plan

• Full test suite passes (421 tests)
• WebVTT-specific tests pass (136 tests)
• from pycaption import WebVTTReader, WebVTTWriter still works
• SCC→VTT produces no STYLE block
• DFXP→VTT produces no STYLE block
• Multiple STYLE blocks merge correctly
• Empty STYLE block produces no output

[github-actions]

🔴 PR Compliance Review

Risk Level: HIGH

• Compliance Issues: 0 (3 critical)
• Regressions: 3

DO NOT MERGE - Critical issues must be fixed first

Full report available in workflow artifacts

[github-actions]

🔴 PR Compliance Review

Risk Level: HIGH

• Compliance Issues: 0 (3 critical)
• Regressions: 3

DO NOT MERGE - Critical issues must be fixed first

Full report available in workflow artifacts

[github-actions]

🔴 PR Compliance Review

Risk Level: HIGH

• Compliance Issues: 0 (3 critical)
• Regressions: 3

DO NOT MERGE - Critical issues must be fixed first

Full report available in workflow artifacts