chore(web): keep the thread details split surface class module-private

[juliusmarminge]

vp run knip:check, the first step of CI's Check job, fails on the V2 branch tip:

Unused exports (1)
THREAD_DETAILS_PANEL_SPLIT_BUTTON_SURFACE_CLASS  apps/web/src/components/chat/threadDetailsPanelStyles.ts

99d76f2 added the constant as an export, but only other constants in the same module use it. This keeps it module-private.

Verification

• vp run knip:check: clean (exit 0).
• apps/web typecheck: no errors or warnings.

Model: Claude Opus 5.5 (Claude Code)

🤖 Generated with Claude Code

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.4 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	2	—	8	✅
Claude	Total thread wire	—	4.9 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.7 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	20.8 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	2	—	8	✅

Baseline: unavailable · PR result: c476160 · 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: 106.1 KiB
• Claude decoded thread snapshot: 106.4 KiB

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

[macroscopeapp]

Approvability

Verdict: Approved at c476160

Macroscope's review found this PR approvable — This one-line change makes an unused styling constant module-private while preserving its value and all consuming class strings. It is a contained encapsulation cleanup with no product-default, runtime behavior, schema, deployment, or static-analysis suppression impact.

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