Drop OSC 52 clipboard writes, force-kill on close, bound the reader, find the ST libs for tests

- OSC 52 is ignored: any output, cat'd or remote, could plant a command
  on the clipboard to paste
- kill(): terminate(force=True), so a child ignoring HUP and INT does
  not outlive its tab with the pty open
- reader pauses once 64 unparsed reads wait, so a large dump blocks the
  child at the pty instead of being buffered whole in memory
- tests/_bootstrap.py: add every Lib/python3* directory; Package Control
  puts the libraries under the active plugin host's own directory

Architecture.md

@@ -100,7 +100,8 @@ are marked ✅.
   truecolor, `ESC[0m` reset. This is what TUIs emit most.
 - **OSC** ✅ — Operating System Command, `ESC ]`, ending at BEL or ST. Carries strings
   rather than numbers: `OSC 0/2` set the window title, `OSC 10/11` query the default
-  foreground/background, `OSC 52` reads or writes the system clipboard.
+  foreground/background, `OSC 52` reads or writes the system clipboard (vterm
+  ignores it: untrusted output must not plant a paste).
 - **DCS / APC** — Device Control String (`ESC P`) and Application Program Command
   (`ESC _`). Long payloads: sixel images, kitty graphics. vterm parses their framing and
   discards the payload, so an image never leaks into the text.