fix: polish switcher overlay spacing and glass

[sonim1]

Summary

• Tighten application icon insets and remove the extra padding below application captions.
• Remove vertical window-tile padding and use matching top and bottom panel insets.
• Keep first and last application captions centered under their icons with symmetric truncation.
• Use NSGlassEffectView on macOS 26 and native popover material on macOS 14–15.
• Add design and implementation records for the approved layout and material choices.

Test Coverage

CODE PATHS                                      USER FLOWS
[+] Directional layout metrics                 [+] Application/window geometry
 ├─ [★★★ TESTED] window insets + height         ├─ [★★★ TESTED] mode-specific sizing policy
 ├─ [★★★ TESTED] app insets + zero bottom      └─ [GAP] live Debug overlay inspection (TCC)
 └─ [★★★ TESTED] supported size scales
[+] Application caption policy
 ├─ [★★★ TESTED] one-column width
 ├─ [★★★ TESTED] first/last edge widths
 └─ [★★★ TESTED] interior width
[+] Native chrome
 ├─ [★★ TESTED] macOS 26 NSGlassEffectView source + Xcode build
 └─ [★★ TESTED] macOS 14–15 popover fallback + deployment compile
[+] SwiftUI wiring
 ├─ [★★ TESTED] directional root padding
 └─ [★★ TESTED] horizontal-only window tile padding

COVERAGE: 10/11 paths tested (91%) | GAPS: 1 manual permission-dependent flow

Tests: 33 → 33 test files (+0); one directional-padding regression test added.

Pre-Landing Review

No issues found.

Design Review

Design Review (lite): 0 findings. AI Slop: clean.

Eval Results

No prompt-related files changed — evals skipped.

Scope Drift

Scope Check: CLEAN. The diff is limited to approved overlay spacing, caption alignment, native glass, tests, and their design/implementation records.

Plan Completion

4 tasks complete, 0 partial, 0 not done. Live Debug overlay inspection was attempted but blocked by macOS Accessibility and Screen Recording permissions for the Debug bundle, as permitted by the verification plan.

Verification Results

• swift build
• swift test — 19 tests, 0 failures
• unsigned Debug Xcode build — BUILD SUCCEEDED
• git diff --check origin/main...HEAD
• Live Debug overlay visual inspection — blocked by Debug-bundle TCC permissions; installed app restored

Documentation

README and project context remain accurate. Approved visual behavior is documented in the design and implementation plan included in this PR.

Test plan

• Verify directional panel inset and tile-size policies across both modes.
• Verify first, middle, last, and one-column application caption widths.
• Verify macOS 26 glass and macOS 14–15 popover fallback integration.
• Run full Swift and unsigned Xcode builds.
• Recheck the signed release overlay after publication on a permission-enabled installation.