OCTO-11588

[OlteanuRares]

• Parse and store the WebVTT line: alignment qualifier (e.g. line:80%,center) via a new LineAlignmentEnum and Layout.line_alignment field
• Map line_alignment to tts:displayAlign in DFXP output (start→before, center→center, end→after)
• Emit the qualifier in WebVTT non-passthrough writer output
• Fix default vertical positioning for multi-line cues when line: is absent — previously all position-only cues used 93.33% (last grid row), pushing multi-line content beyond the viewport. Now uses (15 - num_lines) / 15 * 100 with a floor of 0%.

Test plan

• Unit tests for each qualifier value (start, center, end) and absent case
• Unit tests for multi-line adjustment (3 lines → 80%, 1 line → 93.33% unchanged)
• Conversion tests: VTT→DFXP emits tts:displayAlign, VTT→VTT non-passthrough roundtrips qualifier
• Explicit line: values are never adjusted by the multi-line fix
• Edge case: cues with >15 lines clamp to 0% (not negative)
• SCC/SAMI/SRT chains: no crash, no spurious output, line_alignment stays None when absent
• Full test suite passes (591 tests)

[github-actions]

🟢 PR Compliance Review

Risk Level: LOW

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

SAFE TO MERGE - No critical issues found

Full report available in workflow artifacts