tui: make the modeline construct wordmark open the minibuffer

[edwin-zvs]

The minibuffer is the fleet's command surface, but its only discoverable entry points were a keybinding you have to already know (C-x x / M-x / :) and a title in the matrix-rain panel that isn't always visible. The construct wordmark at the head of the modeline is present in every layout, window size and view mode — which makes it the stable place to anchor a permanent affordance for it.

What changes

• Hover the wordmark → tooltip naming the minibuffer and the keybinding.
• Click → toggles the minibuffer panel.
• The wordmark gets the same accent/underline hover styling the model indicator already uses, so it doesn't read as inert text.

Hover, rendered

   ┌────────────────────────────────────┐
 ─⏸│ Minibuffer — click to open (C-x x) │
   └────────────────────────────────────┘
 construct  focus:view  "s1"  -        theme:matrix | ○ remote | 0.17.5+…
C-x C-f new   C-x b switch   C-x f fork   help

No video: vhs tapes are keyboard-driven and can't move the pointer, so a hover affordance isn't recordable with the standard recipe. The frame above is the real TestBackend render, captured from a throwaway test and reproduced by modeline_wordmark_hovers_a_tooltip_and_toggles_the_minibuffer.

Two judgment calls

Toggle, not open. open_prompt_for_command rebuilds the prompt from scratch (input: String::new(), scrollback reset). Binding the click straight to KeyAction::OpenCommandPalette would mean a second click on an already-open panel silently discards whatever you'd typed. Toggling also matches the existing matrix-panel title click, so the two minibuffer pointer controls agree. The test covers this case explicitly.

A dedicated hit rect, not a HintZone. A HintZone would have given click dispatch for free, but only the open path — and the tooltip needs the region's exact geometry to anchor to. This mirrors modeline_theme_hit / modeline_model_hit.

The wordmark's painted text and its hit rect both derive from one MODELINE_BRAND constant, so the clickable region can't drift off the word it labels; a test asserts the rect starts on the painted word and covers exactly it.

Notes

• Adds specs/0200-modeline-wordmark-opens-minibuffer.md.
• The modeline row now has clickable regions at both ends. They don't overlap, and the pre-existing first-match-wins warning at ui.rs still holds for anything added later.
• Only crates/cli is touched → the relevant binary is construct: .claude/worktrees/modeline-brand-opens-minibuffer/target/debug/construct
• cargo test --workspace --exclude construct-e2e green (1400 CLI tests + the rest); clippy clean; new code is rustfmt-clean (the repo's 61 pre-existing fmt diffs on main are untouched — no blanket cargo fmt).