OCTO-11559

[OlteanuRares]

• Fix fit_to_screen() producing negative/zero extent when origin is at or beyond 90% horizontal / 95% vertical — adds an early-return guard that preserves the layout unchanged
• Fix Size.from_string() rejecting negative values like "-5%" by adding optional minus sign to the regex
• Add VW/VH viewport units to UnitEnum so DFXP files with tts:origin="10vw 10vh" parse without error; as_percentage_of() converts them to % so they never leak into output
• DFXP writer now relativizes the language-level layout_info (div region), fixing duplicate regions in output

Test plan

• Size.from_string("-5%") → Size(-5, PERCENT)
• Size.from_string("-2.5px") → Size(-2.5, PIXEL)
• Size.from_string("80vw") → Size(80, VW)
• Size.from_string("50vh") → Size(50, VH)
• fit_to_screen() returns layout unchanged when origin.x >= 90% or origin.y >= 95%
• Origins below thresholds still get extent adjustment (regression guard)
• Non-percent units (VW/VH) bypass fit_to_screen without crash
• VTT position:95% line:95% → DFXP writer → DFXP reader round-trip succeeds
• DFXP with tts:extent="80vw 50vh" reads without error, writes as % with relativize

[github-actions]

🟢 PR Compliance Review

Risk Level: LOW

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

SAFE TO MERGE - No critical issues found

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