Implement clipboard outbound push

[jserv]

Make the CLIPBOARD selection two-way: when an X client claims ownership, mirror its text out to SDL's clipboard so external host apps can paste it.

A hidden InputOnly requestor window fires an async SelectionRequest for UTF8_STRING at the new owner; convertEvent routes replies destined for that window into clipboardConsumeInternalEvent, which pushes the text via SDL_SetClipboardText. Large payloads arrive via INCR: an INCR-typed reply acks by deleting the property and accumulates PropertyNotify chunks into a 128 MiB-capped buffer, pushing on the zero-length terminator. When an owner declines UTF8_STRING, retry legacy XA_STRING once for the current owner.

Stale-reply defense: each fetch draws its receiving property from a small rotating pool, so a superseded fetch's late reply lands on a different slot and is ignored; declines are discriminated by target. Ownership expiry rides the native SDL_CLIPBOARDUPDATE event: an external host clipboard change (distinguished from our own echo via the last pushed text) revokes the X owner. A clipboard update during the first fetch, before anything has been pushed, does not expire the owner it is fetching from.

Image and other non-text targets are text-only by design (SDL2 has no image clipboard and the SDL2-spelled shim does not wrap SDL3 SDL_SetClipboardData); the dropped target is logged by name.

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Summary by cubic

Mirror X11 CLIPBOARD to the host: when an X client claims CLIPBOARD, we fetch its text and push it to SDL so external apps can paste. Supports UTF8_STRING, XA_STRING fallback with Latin‑1→UTF‑8, and INCR streaming with a 128 MiB cap, with safeguards against stale replies, non‑text data, and echo loops.

• New Features

  • Outbound mirroring via a hidden InputOnly internal requestor; async SelectionRequest for UTF8_STRING, retry XA_STRING once if declined (Latin‑1 transcoded to UTF‑8).
  • INCR support with chunk acks (property deletes), reassembly, and a 128 MiB cap; also caps single‑shot replies; zero‑length terminator triggers push; malformed INCR hints are dropped.
  • Stale‑reply defense using a rotating pool of private properties and a fetch‑pending gate; late or unsolicited replies are ignored.
  • Host clipboard changes (SDL_CLIPBOARDUPDATE) revoke the X owner unless it’s our own echo; the first‑fetch update doesn’t abort the fetch.
  • Non‑text or wrong‑format targets are skipped by design; the dropped target is logged.

• Refactors

  • Internal window plumbing: added createInternalWindow and a Window.internal flag; internal windows don’t emit Create/DestroyNotify and are hidden from XQueryTree and state snapshots.
  • Event routing: convertEvent now calls clipboardConsumeInternalEvent for the requestor window and clipboardHandleExternalUpdate on SDL_CLIPBOARDUPDATE.

^{Written for commit f84b8f1. Summary will update on new commits.}