Fix estimated popup vertical alignment

[FuJacob]

Summary

Anchor estimated-geometry suggestion popups to the resolver's text-line box instead of the full input-field chrome. This removes the extra one-row drop in tall single-line controls such as Chrome's omnibox while preserving field-bottom placement for multiline AX fallbacks.

Validation

• swiftlint lint --quiet — exit 0.
• xcodebuild -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS' -derivedDataPath build/DerivedData CODE_SIGNING_ALLOWED=NO build — BUILD SUCCEEDED.
• xcodebuild -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS' -derivedDataPath build/DerivedData CODE_SIGNING_ALLOWED=NO -jobs 1 build-for-testing — TEST BUILD SUCCEEDED.
• xcodebuild -project Cotabby.xcodeproj -scheme Cotabby -destination 'platform=macOS' -derivedDataPath build/DerivedData CODE_SIGNING_ALLOWED=NO test-without-building -only-testing:CotabbyTests/MirrorOverlayLayoutTests — TEST EXECUTE SUCCEEDED, 20 tests, 0 failures.
• No post-change runtime screenshot was captured; the pure panel-frame regression tests cover both the reported tall single-line field and the preserved multiline fallback.

Linked issues

None.

Risk / rollout notes

Low-risk presentation-only change. Estimated single-line popup cards move upward to the centered caret line; multiline and full-frame fallbacks keep the previous field-bottom anchor. No settings, schema, project, or runtime changes.

Greptile Summary

This PR adjusts estimated mirror popup placement for text fields. The main changes are:

• Popup cards now use the estimated caret line when available.
• Multiline and full-frame fallbacks keep the previous field-bottom placement.
• Layout tests cover the centered single-line case and multiline fallback case.
• Related comments were updated to describe the new placement rule.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
Cotabby/Support/MirrorOverlayLayout.swift	Updates estimated mirror popup placement to use a non-empty caret line before falling back to the input frame.
CotabbyTests/MirrorOverlayLayoutTests.swift	Replaces the old estimated-field placement expectation and adds coverage for the multiline fallback.
Cotabby/Services/UI/OverlayController.swift	Updates mirror-mode comments to describe placement under the best available text line.
Cotabby/Support/CompletionRenderModePolicy.swift	Updates render-policy comments for estimated geometry and popup placement.

_{Reviews (1): Last reviewed commit: "Fix estimated popup vertical alignment" | Re-trigger Greptile}