fix(clipboard): restore internal-clipboard paste fallback; add Termux clipboard

[sinelaw]

Paste via Ctrl+V and the command palette stopped working on Termux (and any
host without a readable OS clipboard) starting in 0.3.12 — issue #2343.

Root cause: the async clipboard-paste rework (#2155) dropped the
internal-clipboard fallback that the old synchronous Clipboard::paste()
had. The background read does arboard::Clipboard::new().get_text(), which
on Termux always fails (arboard has no Android backend). The inline handler
received Ok(None) and returned without inserting anything and without
consulting Fresh's own internal clipboard, so an in-editor copy → paste
round-trip silently did nothing. Pre-0.3.12 the synchronous path fell back
to the internal buffer, so it worked regardless of display server.

Fix:

• The paste thread now returns system_read.or(internal_snapshot), captured
  at dispatch — restoring the in-editor round-trip on Termux, headless TTYs,
  and any environment where the OS clipboard is unreadable. None now means
  "nothing anywhere", so the inline, slow-path, and deadline-cancel branches
  stay correct with no further changes.
• Add real Termux clipboard support via the termux-clipboard-get/-set
  helpers (termux-api), wired through a shared read_system_clipboard() and
  the copy path, so text copied in/out of other Android apps works too. The
  helpers are gated on is_termux() and are inert on every other platform.

The async path gains a test-only system-clipboard reader seam so the
regression can be reproduced deterministically (stub returning None while
the system clipboard stays enabled) without touching the real host
clipboard. The new e2e test copies inside Fresh, pastes with Ctrl+V, and
asserts the internal text lands; it times out without the fallback and
passes with it.

https://claude.ai/code/session_01MrQuX7R9wcsYL4pUDehHay