v2.7.0 - Himinbjörg — one focus + scroll primitive for the TUI

What's Changed

• Himinbjörg: one focus + scroll primitive for the TUI by @kinncj in #7

Himinbjörg — one focus + scroll primitive for the TUI

Scrolling was implemented three times in the TUI and behaved differently in each
place; the full-screen top view (t) didn't scroll like the rest, selection lists
let the cursor drift off-screen, and the mouse wheel did nothing in the top view.
This introduces one shared primitive and moves the top view onto it.

Story: docs/stories/himinbjorg-focus-scroll-20260702235455-0021/Story.md
Design: docs/specs/himinbjorg-focus-scroll/design.md · ADR: docs/architecture/0023-tui-focus-scroll-pane-primitive.md

What's in it

• New app/internal/tui/pane package — the primitive:
  • Pane: scroll, selection that follows the cursor, / filter, sort cycle,
    ▲/▼ more · y/total affordances, focus ring by border weight (survives NO_COLOR).
  • Group: grid layout (side-by-side panels), reading-order Tab, two-level
    page+pane scroll, pointer-targeted mouse, and a whole-page-first focus stop.
  • Capability interfaces (Filterable/Sortable) are opt-in — static panels get neither.
• Top view (t) runs on the primitive. The whole view is the first focus (↑/↓ +
  wheel scroll the entire screen at any resolution); Tab steps the ring through each
  panel and wraps back; wheel/click target the panel under the pointer (was dead here);
  the wide 2-column grid is kept. topview's own scrollWindow copy is deleted.
• Selection stays in view in the command / log-source / sort pickers (keepSelVisible).
• Search / filter, one idiom (/), extended to every list: command picker (by
  name), command result (by output line), and the process view (by command) — on top
  of the existing log search. Each clears with esc and keeps the cursor in view.
• Docs: new docs/guides/18-using-the-dashboard.md (drive the TUI after setup +
  a full keybindings table), corrected 12-top-view.md, updated in-app help (?).

Verification

• 303 unit tests green across 29 packages (go test ./...); go vet clean.
• New tests cover the pane math (clamp, keep-visible, page-auto-scroll), grid focus +
  mouse hit-testing, whole-page focus, and every new filter.
• Top view verified with a headless render (grid alignment, focus ring, whole-page +
  page scroll).
• Gates: Karpathy 87 (PASS_WITH_APPROVAL), a11y PASS (0 critical/serious,
  NO_COLOR-safe).

Deferred (tracked as a follow-up)

Migrating the remaining dashboard modal bodies onto pane.Group so the dashboard's
own scrollWindow copy can be deleted and the Filterable/Sortable API gets a
direct consumer. It's internal dedup with no user-facing change (log/cmd/process
filters already work via the dashboard's own idiom), so it's kept out of this PR to
avoid a large rewrite of a stable view. See the follow-up issue.

Full Changelog: v2.6.3...v2.7.0