WIP: restore lost C behaviors (schtick message, forced redraw, greeting)

Incomplete checkpoint, preserved after the implementing session hit a
capacity limit partway through mutation verification. NOT reviewed and NOT
gate-clean: make fmt has not been run, so fmt-check currently fails on
ARCHITECTURE.md and TODO.md.

Work still outstanding before this is fit for review:
  - run make fmt and fold the result in
  - finish mutation-proving each of the three behaviors
  - verify every message string byte-for-byte against the C sources
  - confirm TestSeedCompatItemTables passes with its golden untouched

Refs #13.

term/tcell.go

@@ -78,6 +78,16 @@ func (t *Tcell) Render(w *game.Window) {
 	t.screen.Show()
 }
 
+// Repaint redraws the whole physical screen from tcell's content buffer
+// — which holds what Render last blitted, so this is C's clearok(curscr,
+// TRUE) + wrefresh(curscr) (command.c, the CTRL('R') arm) rather than a
+// fresh draw of stdscr. Sync throws away tcell's record of what the
+// terminal is showing, so unlike Show it repaints cells it believes are
+// already correct, which is what makes it fix a corrupted screen.
+func (t *Tcell) Repaint() {
+	t.screen.Sync()
+}
+
 // ReadChar blocks for the next key, translated to the byte codes the C
 // game reads: arrows become hjkl, control keys their C0 codes. ok is
 // false when Interrupt woke the read instead of a key arriving.