Vim window tree: splits, ctrl+w chords, multi-window rendering (phase 2)

[gammons]

Stacked on #81 (command mode + ctrl+w reclaim). Merge that first; this PR's base is window-management-phase1.

Summary

• New pure-data internal/ui/wintree package: vim-style split tree with exact-tiling geometry, simulate-and-check minimum-size refusal (validates actual resulting rects, rolls back), Close/Only/Cycle, and geometric ctrl+w h/j/k/l neighbor lookup
• :sp / :vsp / :q / :only (:on) commands plus the full ctrl+w chord set (s v h j k l w q c o) behind a pending-key state with a status-bar hint — disarmed centrally on any mode change
• Multi-window rendering: the focused window renders the real (cached) messages pane at its rect; unfocused windows render placeholder panes (Phase 3 makes them live). Single-window rendering is byte-identical to today — test-pinned
• Phase 2 semantics: one live model; focusing a window on a different channel re-dispatches the standard channel selection (documented interim, replaced by per-window models in phase 3)
• Hardening from review: workspace-switch window-tree reset (cross-workspace state leak), resize-after-split crash clamps, interim mouse guard in split mode (full routing is phase 4)

Spec: docs/superpowers/specs/2026-06-11-window-management-design.md (§1, §4-6, Phasing item 2); plan: docs/superpowers/plans/2026-06-11-window-management-phase2-window-tree.md (on main).

Test Plan

• 27 wintree unit tests (tiling incl. odd/offset bounds, refusal incl. nested-sibling regression, rollback exactness, navigation overlap rules)
• App-level tests: commands, chords (incl. esc-mid-chord swallow, hint lifecycle, mode-change disarm), workspace-switch reset, hard-shrink crash regressions, split-mode mouse guard
• Full suite green (go test ./... -count=1), vet clean, every commit builds
• Manual smoke: :vsp → side-by-side with live focused pane + placeholder; ctrl+w h/l moves focus and live pane follows channel; :q/ctrl+w o collapse; shrink until "Not enough room"; ? lists window commands

[gammons]

Superseded by #87, which combines all three window-management phases into one mergeable PR.