harness: split native/harness/src/ui.zig into src/ui/*

[btipling]

Summary

native/harness/src/ui.zig is 1637 lines. frame() alone is ~580; paintToolRun ~240; paintThinking ~120. Product paint still lands here after earlier extractions (busy_row, chip_preview, model_picker, transcript_split, thinking_collapse, rect_spinner).

A src/ui/ directory is feasible. main.zig only imports ui.zig (onInit / onDeinit / frame / BUILD_ID). Split is a mechanical move of already-bounded fns — not a redesign.

Why this works

• Paint helpers are already named and mostly take src + args (paintToolRun, paintThinking, paintLastUserChip, paintStatusSlots, paintSkillAttached, paintStatusChip).
• Same pattern as #607 / rect_spinner.zig and #645 / chip_preview.zig.
• Zig allows src/ui.zig and src/ui/*.zig as siblings (@import("ui/toolrun.zig") from the facade).
• build.zig does not special-case ui.zig by path.

Hard part (do not ignore)

File-level vars couple the paint: transcript_scroll, tool-run / thinking open maps, composer_last_h, chip (msg_content_y, last_user_slot, prev_chip_visible), prompt_buf. Those need a single src/ui/state.zig (or stay on the facade and get passed in). Do not clone globals per file.

Proposed layout

Keep src/ui.zig as the public facade (onInit, onDeinit, frame, BUILD_ID). Move bodies under src/ui/:

File	From ui.zig
ui/state.zig	file-level vars + resetTranscriptScroll / open-map clears
ui/scroll.zig	isNearBottom, clampScrollToContent, scrollToBottom
ui/kinds.zig	kindLabel, kindTextColor, kindFill, lifecycleLabel
ui/chrome.zig	chrome*Font, paintStatusChip, toolRunClipboard
ui/toolrun.zig	paintToolRun, isCommandLikeRun, detailUsesMono
ui/thinking.zig	paintThinking, thinkingPreview
ui/chip.zig	paintLastUserChip
ui/status.zig	paintStatusSlots, truncateStatusValue, truncateToWidthPx, utf8CharLen, statusPackMaxWidth
ui/skill.zig	paintSkillAttached
ui/composer.zig	composer constants, clearPrompt, submitText

frame() stays on the facade until a later pass. Do not split it in the first PR unless it is still the only thing in ui.zig and still huge.

Goal

• Same pixels, same ids, same one-frame settle. Operator-visible behavior unchanged.
• main.zig import path unchanged (@import("ui.zig")).
• Each moved file stays dvui-paint only — no new bridge protocol, no DOM.

Non-goals

• Behavior / palette / protocol changes.
• Rewriting frame() layout math.
• Moving already-extracted modules (busy_row, chip_preview, model_picker, …) into ui/.
• src/ui/root.zig rename (keep the facade file so import sites stay put).

Proof

• zig fmt on the new tree.
• Existing harness tests still green (test-rich, layout tests that import busy_row / picker — they should not need ui.zig).
• build-harness artifact still loads; smoke: composer hug, status pack, tool-run expand, thinking collapse, last-user chip, stick-to-bottom.

Suggested next: create-plan (one PR, mechanical).

[btipling]

Plan: #656 (create-plan, DRAFT). Next: plan-review on #656, then implement.

[btipling]

Done: split shipped in #658. monolithic ui.zig (1628 lines) is now a facade + 11 ui/*.zig modules.