fix: serialize NSPasteboard access to stop the ⌥C / clipboard-poll AppHangs

[soyasis]

What

Routes every NSPasteboard access through a single serial queue so clipboard reads no longer block the main thread — fixing the 1.5.0 AppHang timeouts (Sentry) while keeping the no-concurrent-readers guarantee.

Why

On 1.5.0 a synchronous pasteboard read on the main thread could block for seconds on pboardd — fetching a Universal Clipboard / Handoff item over the network, or a large or promise-backed item — freezing the ⌥C hot path and the clipboard-history poll. The previous "everything on main" approach avoided a separate crash (two concurrent readers crashing in __NSFastEnumerationMutationHandler) but is exactly what left the main thread exposed to those hangs.

How

• New PasteboardQueue: a process-wide serial DispatchQueue that funnels every pasteboard content read/write. Reads are async (off-main, so a slow daemon never hangs the runloop), writes are fire-and-forget but still serialized, and flush() drains pending writes on quit so a copy made right before ⌘Q isn't lost.
• The ⌥C / history-poll detection takes ONE atomic snapshot per call (file URLs + text + image + changeCount in a single hop) and resolves off the lane — OCR and disk loads run after the read returns. A write landing mid-detection can no longer make the window act on a mix of two clipboard states, and Vision OCR doesn't occupy the shared lane.
• Cheap changeCount probes stay on the caller's thread (the property is cached and doesn't iterate the items array).

Testing

• Build + unit tests pass.
• Manual matrix on a real device: plain text, URL, image file (with and without OCR-able text), screenshot image data, empty clipboard, clipboard-history re-copy, copy-then-⌘Q, a ~23 MB clipboard, and a rapid ⌥C + paste-back stress pass (no crash, no hang).
• Universal Clipboard / Handoff — the original freeze trigger — ⌥C no longer hangs.

Notes

• Paste-back holds the serial lane for ~0.4s after each paste (its restore window). It's bounded and only delays a ⌥C fired immediately after a paste-back — kept deliberately, because freeing the lane would let a back-to-back paste-back restore the wrong clipboard.
• Re-copying a clipboard-history entry floats it to most-recent (standard clipboard-manager behavior).