Rebind char cycling to ,/. instead of [/]

[ and ] needed a reach/shift on most layouts and felt awkward next to
the otherwise all-unshifted control scheme (hjkl, space, digits). ,
and . are adjacent, unshifted, easy to hit without looking away from
the canvas -- a natural prev/next pair. Old bindings kept as a fallback
rather than removed.

Verified both in isolation (Update() with a KeyMsg) and live over a
real SSH session -- the status-line ink swatch visibly cycles from █
to ▓ on a fresh connection. An initial live test after the rebind
appeared to fail, but that traced to a stale session left over from
the server restart, not an actual bug -- confirmed working correctly
on a fresh connection.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -18,7 +18,7 @@ fresh enough to reuse without re-learning it.
 - Arrow keys or `hjkl` — move your cursor
 - `space` / `enter` — paint
 - `backspace` / `delete` — erase
-- `[` / `]` — cycle the ink character
+- `,` / `.` — cycle the ink character (`[` / `]` also work)
 - `1`-`8` — pick a color
 - `c` — clear the whole canvas (yes, for everyone — it's a shared wall)
 - `q` / `ctrl+c` — quit