Add active border drawing to RulerWindow and RulerContentView

[pascalpp]

Summary

• Draw a 1px inset active border around the active ruler panel using the existing L-shaped/one-wing ruler geometry.
• Keep the active border synced as rulers are created, activated, restored, cycled, and closed.
• Use a 25% opacity active border so it stays visually secondary to the existing 50% outer border.

Tests

• xcodebuild -project "Free Ruler.xcodeproj" -scheme "Free Ruler" test -only-testing:FreeRulerTests
• xcodebuild -derivedDataPath /private/tmp/freeruler-active-border-derived-data -project "Free Ruler.xcodeproj" -scheme "Free Ruler" test -only-testing:FreeRulerTests/RulerCoreTests/testRulerManagerDrawsActiveBorderOnlyOnActiveRuler
• git diff --check

Fixes #260

[pascalpp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• Add active border drawing to RulerWindow and RulerContentView #261 👈 (View in Graphite)
• feature: Support Multiple Rulers #246 : 9 other dependent PRs (#238 , #253 , #255 and 6 others)
• main

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[pascalpp]

[codex reviewer] Reviewed this against codex/251-remove-legacy-ruler-window.

No blocking findings. The active-border state is centralized in RulerManager activation/close/restore paths, old controllers have their border cleared during restore, and the paint path stays inside the existing border view. The added test covers create, active handoff, and close fallback to the remaining ruler.

Verification: xcodebuild -project "Free Ruler.xcodeproj" -scheme "Free Ruler" build CODE_SIGNING_ALLOWED=NO passes on the cumulative stack. The normal focused unit-test command could not complete locally because Xcode failed while signing the app due the nested FreeRulerTests.xctest bundle being unsigned; retrying with signing disabled launched an unsigned test host that exited before XCTest connected.