Refactor NotePanel: cache derived tags/links/backlinks and avoid hot-path IO

[multiplex55]

Motivation

• Reduce CPU and IO work in the UI hot path by avoiding repeated recomputation of tags, wiki/markdown links, and backlink rows during repaint.
• Avoid per-keystroke or per-segment file reads (todos/notes) by moving expensive operations off the render path and using cached snapshots.
• Ensure derived-linking state is refreshed at well-defined semantic points (panel init, successful save, and explicit actions) rather than on every edit event.

Description

• Added a NoteDerivedView struct and new NotePanel fields (derived: NoteDerivedView, derived_dirty: bool, and a test-only derived_recompute_count) to cache derived metadata such as tags, wiki_links, external_links, backlink rows by category, and a todo_label_map for markdown preprocessing in src/gui/note_panel.rs.
• Centralized recomputation in NotePanel::refresh_derived(), called on panel init and after successful saves (also used for overwrite/save-as-new paths), and introduced backlink_rows_for_active_tab() to page over precomputed backlink rows instead of recomputing each render.
• Modified preprocess_note_links to accept an injected todo label map (todo_labels: &HashMap<String, String>) to remove direct load_todos() IO from markdown preprocessing and wired all callers to use self.derived.todo_label_map.
• Added a cheap read-only snapshot accessor pub fn note_cache_snapshot() -> Vec<Note> in src/plugins/note.rs so UI code can read cached notes without calling load_notes() (disk) during repaint.
• Mark derived_dirty = true on edit-mode changes and when rendering segments mutate the note, and avoid per-keystroke recomputation; refresh_derived() runs after a successful save() (and related save flows) to update self.note.links and backlink rows.
• Added unit tests: in src/gui/note_panel.rs tests for derived reuse across renders, recomputation after save() and note.links update, backlink invalidation when slug/targets change, and preprocess_note_links using an injected todo map; in src/plugins/note.rs tests for note_cache_snapshot read-only copy semantics and that it reflects refresh_cache() updates.

Testing

• Ran cargo fmt --all successfully to format changes.
• Attempted to run the note_panel tests with cargo test, but the environment Rust toolchain is older than required by a transitive dependency (error: rustc 1.87.0 vs time requiring >=1.88.0), so the test suite could not be executed here.
• New unit tests were added to src/gui/note_panel.rs and src/plugins/note.rs (see PR diffs) and should pass on a compatible toolchain when run (cargo test).

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