refactor(web): self-contain composer context strip visibility in BranchToolbar (#436)

[rynfar]

Closes #436

Summary

Previously, ChatView mirrored the internal visibility rules of every child inside BranchToolbar (showGitControls, showEnvironmentIndicator, hostsRestingComposerControls, restingComposerControlsVisible, hasCapacityReading) to decide whether to mount and display the composer context strip. This duplication broke whenever children changed their rendering rules or during rollbacks.

This PR refactors BranchToolbar to self-contain its content measurement and visibility:

• BranchToolbar measures its laid-out DOM children via resolveContextStripHasContent and reports changes upward via onContextStripVisibilityChange.
• Provides resolveInitialContextStripVisibility to avoid layout shift on initial render.
• Simplifies ChatView by removing duplicate child flags and resolveComposerContextStripVisibility.
• Updates tests accordingly.

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Project	Deployment	Actions	Updated
pylon-marketing	Ignored	Preview	Sep 21, 2026 10:44pm UTC

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.9 KiB	14.0 KiB	+20 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	+2 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	+18 B (+0.3%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	14.0 KiB	13.9 KiB	−35 B (−0.2%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	−7 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.8 KiB	6.7 KiB	−28 B (−0.4%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: a3d6f94 · PR result: e83fde4 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 115.6 KiB
• Claude decoded thread snapshot: 116.3 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.