UI: Paste button and menus paste the system clipboard; freeze regression test (#273)

In 0.5.0 a click anywhere on the Home ribbon's Paste button (label,
icon or dropdown arrow) froze the app for good. home() called
paste.secondary_clicked() inside ctx.data_mut(); after any click egui
answers that with ctx.input(), which takes the context lock the closure
already holds, so the UI thread waited on itself. Sampling the hung
0.5.0 process on macOS shows the main thread parked in
RawRwLock::lock_exclusive_slow under Context::write <- data_mut <-
ribbon::home. #272 moved the query out of the closure; it landed after
the 0.5.0 release, which is the build both reporters used.

No test ever clicked the Paste button itself, which is how this
shipped. The new test clicks it, then opens its menu with a secondary
click and picks Paste Values, on a worker thread with a timeout so a
regression fails instead of hanging the run. With #272 reverted it
fails (egui's debug lock check panics after 10 s); with it, it passes.
The Paste button gets an accessible "Paste" name so the test (and
screen readers) can find it, the same way Pictures and Border presets
are named.

With the freeze gone, the button still did not do what it says: it
pasted GridCraft's own copy only (GridState::system_clipboard was never
assigned), so text copied in another app gave "Nothing to paste." The
ribbon button, its options menu's Paste, the cell and header context
menus and the macOS Edit menu now read the system clipboard through a
new paste_text host service and paste it like Cmd/Ctrl+V does, falling
back to GridCraft's own copy when there is no text (or no service, as
on the web). Asking egui for the clipboard instead (RequestPaste) was
tried and stalled the macOS event loop until the next input event.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ui-parity.md

@@ -1,6 +1,6 @@
 # UI parity
 
-> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-11 · **Change:** minor (AutoFit reserves filter dropdown space, observed against Windows Excel) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-10 · **Last updated:** 2026-10-12 · **Change:** minor (ribbon and menu Paste paste the system clipboard like Cmd/Ctrl+V; regression test for the Paste button freeze, #273; Paste button gets an accessible name) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Interaction, controls and feel, against Excel. Part of [target-app-parity.md](target-app-parity.md).
 **UI/UX fidelity: ~55% ready, 50–80 h** (estimated). Ribbon command coverage is measured
@@ -160,6 +160,7 @@ scrollbar bugs 2–4 · pixel/feel pass against observed Excel 7–12.
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-12 | minor | Paste from the ribbon (button and options menu), the cell and header context menus and the macOS menu bar's Edit menu pastes what is on the system clipboard, as Cmd/Ctrl+V does, instead of only GridCraft's own copy (the web build, which can't read the clipboard on request, still pastes GridCraft's copy). The desktop app reads the clipboard off the UI thread and gives up after 5 s, so a clipboard source that never answers can't freeze it. Regression test for the Paste button click freeze in 0.5.0 (#273, fixed by #272); the Paste button gets an accessible "Paste" name for screen readers |
 | 2026-10-11 | minor | AutoFit accounts for table and ordinary filter dropdowns, including low zoom; header text excludes the button area |
 | 2026-10-11 | minor | Outline band for grouped rows and columns, Show / Hide Detail on the Data tab, numpad navigation keys in shortcuts (#373) |
 | 2026-10-11 | minor | Shift+Space / Ctrl+Space no longer start an edit; Control+Space selects the column on Mac (#338) |