fix: prevent mask coverage gap during adjacent segment transitions

[AdamJBurns]

When two mask segments are directly adjacent (share a boundary) and a zoom is applied, the current interpolation logic can fade out/in at the shared edge. During the transition, opacity briefly drops at that boundary, exposing sensitive content.

All other behavior is unchanged: PreparedMask construction, clamping, feathering, and size handling remain identical.

Fixes: #1619

Greptile Summary

Prevents opacity gaps during adjacent mask segment transitions by detecting when segments share boundaries and skipping fade-in/fade-out at those points

• Collects enabled segments upfront to enable index-based iteration
• Checks for adjacent segments before and after current segment using time boundaries
• Sets fade multipliers to 1.0 when adjacent segments detected, maintaining full opacity
• Found minor threshold issue: using < instead of <= may miss segments exactly at fade_duration apart

Confidence Score: 4/5

• Safe to merge with one minor threshold precision fix recommended
• Core logic correctly detects adjacency and prevents fade gaps, but comparison operator should use <= instead of < to catch exact boundary matches at fade_duration distance
• crates/rendering/src/mask.rs - fix adjacency threshold comparison operators on lines 93 and 98

Important Files Changed

Filename	Overview
crates/rendering/src/mask.rs	Added adjacency detection to prevent fade gaps at mask boundaries - logic is sound but has a minor threshold precision issue

_{Last reviewed commit: 1354425}

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