feat(web): use OKLCH for theme palettes

[rynfar]

Moves palette generation to OKLCH so generated theme ramps are perceptually uniform. Rewrites themePalette.ts (+592), index.css, and the boot script in index.html, and updates ThemeColorPicker and the VS Code theme importer.

Adds two dependencies: culori and @types/culori.

Cherry-picked from upstream 083fa4ab24c464ddf01e5b7ab22135d1ebdc120b (pingdotgg/t3code#6036). Clean, despite five files where Pylon had diverged.

Why it applies cleanly: #6183 (theme-aware environment artwork) is an ancestor of this commit, and Pylon adopted that earlier today. Taking these out of order would have fought.

index.html checked Pylon-first. The diff adds no visible T3 branding and leaves <title>Pylon (Alpha)</title> untouched. The t3-chat-dark theme id and the boot-script comments that mention T3 Chat were already on pylon (5 references before this change, none added) and are compatibility identifiers — renaming them would orphan saved themes.

Also carries the ledger entry for this batch, covering both this PR and #2, and advances reviewed-through to 2db08457f. DEF-1 and DEF-2 re-evaluated and still deferred: the date gate is 2026-08-25 and the churn check is non-empty.

Verification

themePalette, themeBoot, vscodeThemeImport, and SidebarStageBackdrop pass (78 tests). Web typecheck, lint, and format clean.

Worth a human eye before merge: tests confirm the math, not the appearance. A color-space change can compute correctly and still shift how the five built-in themes look, so this deserves one pass in a real client.

Model: Claude Opus 5 (1M context). Harness: Claude Code.

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[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	11.4 KiB	11.3 KiB	−4 B (−0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	5.5 KiB	5.5 KiB	+2 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	5.9 KiB	5.9 KiB	−6 B (−0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	49.7 KiB	49.7 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	16	16	0 (0.0%)	21	✅
Claude	Total thread wire	11.4 KiB	11.4 KiB	+3 B (+0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	5.5 KiB	5.5 KiB	+4 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	5.9 KiB	5.9 KiB	−1 B (−0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	50.6 KiB	50.6 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	16	16	0 (0.0%)	21	✅

Baseline: c53f5b8 · PR result: 10ff060 · 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: 94.6 KiB
• Claude decoded thread snapshot: 95.4 KiB

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