feat(navigation): add canonical file tree browser primitive

[ty13r]

Summary

• Rebuilt PR feat(navigation): add canonical file tree browser primitive #127 on current main as a canonical baseline navigation primitive.
• Adds UnifiedIUR.Widgets.Navigation.file_tree_browser/1 with attributes.file_tree and recursive folder/file-leaf normalization.
• Keeps :file_tree_browser out of UnifiedIUR.Widgets.Components, UnifiedUi.WidgetComponents, and AshUI.WidgetComponents.
• Wires Ash UI admission and fallback rendering, Live UI native rendering, and Desktop UI / Elm UI native mappings.
• Documents the public widget type and adds focused tests across the package boundaries.

Architecture Decisions

• Canonical name: file_tree_browser, generic and repo-independent.
• Family: baseline :navigation, not layer shell/callout and not widget-component catalog.
• Canonical attributes: attributes.file_tree with tree_id, root_label, nodes, selected_path, default_expanded?, selection_intent, and toggle_intent.
• Interactions: Live UI synthesizes canonical selection and change interactions from semantic intents. It does not expose raw on_select / on_toggle event names in canonical data.
• Presentation: canonical leaf nodes may carry file_kind, language, and line_count; renderer-specific glyphs/markers/indent are local presentation concerns.
• State: expansion remains host-owned through the semantic toggle intent, so runtime state does not move into the canonical tree model.

Open Questions Answered

1. :tree_view extension vs new kind: use a new baseline primitive. Filesystem navigation has different semantics from generic data trees.
2. Family assignment: use baseline :navigation.
3. Glyph source: keep glyphs out of canonical data; renderers can derive display from file_kind/language/path.
4. Folder icon: renderer/theme concern; v1 uses an expand/collapse marker only.
5. Expand/collapse state model: host-owned via toggle_intent.
6. Map FilesPane migration: follow-up data-shaping integration after the primitive lands.
7. Drag/drop semantics: deferred follow-up, not part of the canonical v1.

Verification

Passing:

• mix test packages/unified_iur/test/unified_iur/widgets/file_tree_browser_test.exs packages/unified_iur/test/unified_iur/widgets_test.exs packages/live_ui/test/live_ui/widgets/file_tree_browser_test.exs packages/live_ui/test/live_ui/renderer_test.exs packages/desktop_ui/test/desktop_ui/renderer_test.exs packages/elm_ui/test/elm_ui/renderer_test.exs test/ash_ui/resource/file_tree_browser_test.exs test/ash_ui/phase_31_package_boundary_test.exs test/ash_ui/phase_31_runtime_adapter_test.exs
• mix format --check-formatted <changed files>
• bash ./scripts/validate_specs_governance.sh

Known unrelated checkout issue:

• mix test packages/desktop_ui/test/desktop_ui/phase_eleven_integration_test.exs still fails one existing release-readiness assertion because DesktopUi.Validate reports missing .spec/specs/desktop_ui/* and .spec/planning/desktop_ui/* traceability files in this checkout. The renderer coverage checks pass.

Spec-loop tasks are not available in this checkout: mix spec.prime, mix spec.next, and mix spec.check all return task-not-found.