implement 9-phase feature completeness plan

Phase 1: Tab/Shift+Tab nav, PgUp/PgDn, Ctrl+Home/End, go-to cell, status bar stats
Phase 2: Batch undo, absolute refs ($), copy/cut/paste, insert/delete rows/cols, find & replace
Phase 3: 30+ formula functions (math, string, info, stats) + SPARKLINE visualization
Phase 4: Number formatting (general, fixed decimal, currency, percentage)
Phase 5: Sort by column (asc/desc), freeze panes
Phase 6: Cell styling (bold, underline, fg/bg colors via commands and Ctrl+B/U)
Phase 7: Command palette (:), system clipboard integration via arboard
Phase 8: Multiple sheets (Workbook), sheet tabs, Ctrl+PgUp/PgDn switching, workbook persistence
Phase 9: Column filtering, cell comments with indicator glyphs, SPARKLINE function

Also cleans up dead code: removes superseded methods, wires replace_all to Ctrl+A,
and eliminates all compiler warnings.

285 unit tests + 17 doc tests, all passing. Zero warnings on release build.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: SamuelSchlesinger

Cargo.lock

@@ -18,6 +18,7 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 csv = "1.3"
 reqwest = { version = "0.11", features = ["blocking"] }
+arboard = "3"
 
 [dev-dependencies]
 tempfile = "3.0"