fix(ios): grok tucked tab returns on focus + generic diff pill when blurred

[boggspa]

Two composer-blur polish items from on-device testing of build 23.

1. Grok tucked tab now returns on focus

All other shells returned their above rows on focus; grok didn't. The focus-gated above-rows group had an implicit zIndex, while the composer core below carries .zIndex(tuckedTab ? 1 : 0). SwiftUI renders a conditionally-inserted sibling behind an explicit-zIndex sibling on insertion — so grok's tab (the only tuckedAboveTab shell) got eaten when it re-appeared on focus. Fix: give the group an explicit .zIndex(0) so both siblings are explicitly ordered.

2. Generic diff pill when blurred + active changes

When the composer is blurred and there's an active run diff, a compact "N files +X −Y" pill (ComposerDiffPill) now floats above the one-line composer — generic for every shell, tap opens the diff — instead of the full composer-specific changes row. The full, shell-specific above rows still return on focus. So during active changes you always get a mini diff indicator above the collapsed composer (Codex-app-style, our twist: muted "N files", green additions, red deletions, compact k counts).

Blurred-state matrix now:

• active diff → generic pill
• no diff → nothing (just the one-line composer)
• focused → full composer-specific above rows (incl. grok's tucked tab)

Verification

swift build ✓ · swift test 73/73 ✓ · iPhone-17 simulator BUILD SUCCEEDED ✓

The grok zIndex fix is a strong root-cause hypothesis (the implicit-vs-explicit zIndex insertion glitch is a known SwiftUI behavior) but, like the rest of the shell work, worth a quick on-device confirm in build 24 since I can't see it render.

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