[P2-28] Add terminal accessibility, search, refresh, and application actions

[phranck]

Problem

TUIkit lacks terminal-meaningful cross-cutting SwiftUI behaviors: accessibility labels/values/hints/hidden state, .searchable, .refreshable, keyboardShortcut, Link, Toolbar families, ScenePhase, OpenURLAction, and DismissAction. Without them, controls and navigation cannot expose a consistent semantic model or application action flow.

Application command menus are already tracked by issue #5 and should consume the same action/shortcut infrastructure rather than being reimplemented here.

Proposed solution

• Add an internal semantic tree populated by SwiftUI-compatible accessibility modifiers and control defaults.
• Add searchable state/presentation for lists, tables, navigation, and command palettes.
• Add refresh actions with task identity, cancellation, and visible progress semantics.
• Add keyboardShortcut resolution and action dispatch shared with Enhancement: Implementation of an Application Menu System #5.
• Add Link, ScenePhase, OpenURLAction, and DismissAction through the per-app Scene/Environment model with portable terminal behavior.
• Add ToolbarContent, ToolbarItem, applicable placements, and toolbar modifiers mapped to status-bar and command regions without duplicating Enhancement: Implementation of an Application Menu System #5.

Acceptance criteria

• Semantic snapshots expose stable labels, values, hints, roles/states, and hidden behavior for all public controls.
• Search filters/queries, focus, cancellation, and navigation are deterministic.
• Refresh runs once per trigger, inherits actor isolation, and cancels on identity removal/shutdown.
• Shortcut conflicts have deterministic priority and are testable without a real terminal.
• Scene/open/dismiss actions and Link are injected per app and have documented unsupported-platform behavior.
• Toolbar content composes with Scene/commands/status-bar placement and preserves stable action identity.
• Swift 6.0 macOS/Linux build, tests, lint, and DocC are warning-free.

Dependencies

Depends on runtime #8, event loop #9, Scene composition #19, data flow #18, focus #22, controls #23, and navigation #31. Command rendering remains in #5.

Parallelization

Accessibility semantics and action/environment infrastructure can proceed in parallel. Search/refresh integration follows the relevant containers.

Commit structure

• Split this issue into small, thematic, independently revertible commits wherever the work can remain coherent.
• Every commit must build and keep its applicable tests/gates green. Do not commit an intentionally failing regression test; use local/known-issue characterization or land the test with the smallest fix.
• Keep characterization/fixtures, mechanical renames or moves, semantic changes, and documentation/migration updates separate when each step remains green.
• The issue boundary is not a commit boundary; multiple commits are expected for independently reversible changes.