OCTO-11587

[OlteanuRares]

• Fix WebVTTReader silently discarding the position: cue setting when line: is not present
• Reader now defaults vertical origin to 93.33% (WebVTT bottom-of-viewport, line:-1 on 15-line grid) so position-only cues produce a valid Layout
• Guard _check_line_overflow to suppress false-positive viewport warnings when line: was not explicitly set

Test plan

• position:70% (no line:) produces Layout with origin (70%, 93.33%)
• VTT→DFXP output includes tts:origin with horizontal position
• VTT→SAMI output includes margin-left with horizontal position
• Both position: + line: present → existing behavior unchanged
• Neither present → layout_info remains None
• VTT→VTT roundtrip preserves position: without injecting synthetic line:
• No false-positive overflow warning for position-only multiline cues
• SCC→VTT→DFXP/SAMI/SCC paths unaffected (SCC always has both settings)
• Full test suite passes (584 tests)

[github-actions]

🟡 PR Compliance Review

Risk Level: MEDIUM

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

REVIEW REQUIRED - Address issues before merging

Full report available in workflow artifacts