feat: shared PTY terminal (M12 Tier 3) + readable form controls

- feat(share): host-consented interactive terminal over the relay. The host shares
  a real PTY shell (core/pty_session.py); the guest renders it with pyte and sends
  keystrokes (gui/terminal_widget.py) — vim/top/tab-completion/Ctrl-C work. Runs as
  the host's user (never root). The host reads along live and can type too, e.g. a
  sudo password, which stays local and is never sent to the guest. Off by default.
  Guest also pulls inventory on join (req_full).
- fix(gui): style all form controls (QLineEdit/QPlainTextEdit/spin boxes/combo/
  terminals) dark-on-light-text — Fusion defaulted them to unreadable light-on-light.
- replaces the command/response shell with the full PTY; adds pyte to the gui extra.

Verified end-to-end against the deployed relay (guest keystroke ran on host PTY).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_pty_session.py

@@ -0,0 +1,27 @@
+"""Tests for the host PTY session (M12 Tier 3)."""
+
+import time
+import unittest
+
+from rigdoctor.core.pty_session import PtySession
+
+
+class PtySessionTests(unittest.TestCase):
+    def test_runs_command_and_reads_output(self):
+        pty = PtySession(rows=24, cols=80)
+        try:
+            time.sleep(0.4)
+            pty.read()  # drain the shell prompt
+            pty.write(b"echo PTY_MARKER_42\n")
+            deadline = time.time() + 3
+            buf = ""
+            while time.time() < deadline and "PTY_MARKER_42" not in buf:
+                time.sleep(0.1)
+                buf += pty.read().decode(errors="replace")
+            self.assertIn("PTY_MARKER_42", buf)
+        finally:
+            pty.close()
+
+
+if __name__ == "__main__":
+    unittest.main()