Split note derived recompute into fast (local) and heavy (global) paths

[multiplex55]

Motivation

• Reduce per-frame work when editing a note by only doing cheap local recomputes immediately and deferring expensive global work (todo-label loading and backlink row construction).
• Avoid rebuilding unchanged global caches during rapid edits while still ensuring backlinks and todo labels update after saves or when the underlying note/todo datasets change.

Description

• Introduce refresh_fast_derived for immediate local recompute of tags, wiki_links, and external_links, and refresh_heavy_derived for expensive global recompute of todo_label_map and backlink rows using note_version() and todo_version() to short-circuit unchanged work.
• Add fast_derived_dirty, heavy_recompute_requested, last_edit_at_secs, last_notes_version, and last_todo_revision fields to NotePanel, and replace the old recompute counter with heavy_recompute_count under #[cfg(test)].
• Trigger fast recompute immediately on content mutations and schedule/debounce heavy recompute with maybe_refresh_heavy_derived using a HEAVY_RECOMPUTE_IDLE_DEBOUNCE constant, while forcing heavy recompute after successful saves and after overwrite/save-as-new flows.
• Update call sites to use mark_content_changed on edits/preview mutations and to call refresh_fast_derived/refresh_heavy_derived(true) where a full refresh is required, and add the unit test edits_do_not_trigger_heavy_recompute_every_frame while adjusting existing derived-recompute tests to track heavy recomputes.

Testing

• Formatted the code with cargo fmt --all which completed successfully.
• Attempted to run the targeted unit test with cargo test edits_do_not_trigger_heavy_recompute_every_frame -- --nocapture, but the build failed in this environment due to a missing system dependency (alsa.pc) required by the alsa-sys crate, preventing the test run from completing.
• Existing derived-recompute unit tests were updated in-source to assert against the new heavy_recompute_count behavior, and the new test was added to prevent heavy recompute increments on every frame during active edits.

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Codex Task