Let a program ask how big the text grid is

`RGR(1)` and `RGR(2)` gave the window in pixels and nothing gave columns, rows
or the cell size -- so anything placing a character *and* a sprite at the same
spot had to hardcode a number measured by hand against whatever font the host
loaded. The Breakout in examples/ does exactly that, `CW# = 16`, and it is the
one thing in that listing that breaks on a different font or window.

**`RWINDOW` is BASIC 7.0's own answer and had never been implemented here.**
`RWINDOW(0)` is the current text window's rows and `RWINDOW(1)` its columns.
`RWINDOW(2)` reports a C128's 40 or 80 column screen mode, and this interpreter
has neither -- refused by name, because answering 0 would be a plausible lie,
which is worse than a refusal that says why.

The cell size in pixels is `RGR(3)` and `RGR(4)`, beside the surface's own
dimensions rather than on `RWINDOW`. Two reasons: a cell size is a fact about
the surface, and `RWINDOW` reports the *window*, so dividing `RGR(1)` by a
column count stops being right the moment a program calls `WINDOW`.

Both read a new optional `grid` entry point on `akbasic_TextSink` -- columns,
rows, cell width, cell height -- implemented by the akgl sink and forwarded by
the tee, in the shape `moveto` and `window` already had. NULL everywhere else,
so both verbs refuse by name against a sink with no grid. `akbasic_sink_init_
stdio()` clears it for the same reason it now clears the other two.

Measured on the standalone build: `RGR(3)` answers 16 and `RWINDOW` answers 50
columns by 37 rows -- the three numbers the Breakout listing had written out as
constants -- and `RWINDOW` follows a `WINDOW` call while `RGR(3)` does not.

tests/console_verbs.c drives the answers through a stand-in sink with a grid,
since the harness sink is stdio and has none; tests/graphics_verbs.c covers the
new `RGR` fields, their refusal, and the moved range bound. The `c excerpt=`
block in docs/10-embedding.md moves with the header, which is `docs_examples`
doing its job.

TODO.md section 6 item 31's second half, struck.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

include/akbasic/sink.h

@@ -61,6 +61,27 @@ typedef struct akbasic_TextSink
      * @return `NULL` on success, otherwise an error context owned by the caller.
      */
     akerr_ErrorContext AKERR_NOIGNORE *(*window)(struct akbasic_TextSink *self, int left, int top, int right, int bottom);
+    /**
+     * Report the character grid, or NULL when the sink has none.
+     *
+     * `RWINDOW` and `RGR(3)`/`RGR(4)` are the callers. Without this a program
+     * had no way to ask how big a character is, so anything placing a character
+     * and a sprite at the same spot had to hardcode a cell size measured against
+     * whatever font the host happened to load -- which the Breakout in
+     * `examples/` did, and it was the one thing in that listing that broke on a
+     * different window.
+     *
+     * Columns and rows are the *current* window, so they follow `window()`;
+     * the cell size does not, because windowing does not change it.
+     *
+     * @param self The sink.
+     * @param columns Output: columns in the current text window.
+     * @param rows Output: rows in it.
+     * @param cellw Output: a character cell's width in pixels.
+     * @param cellh Output: its height in pixels.
+     * @return `NULL` on success, otherwise an error context owned by the caller.
+     */
+    akerr_ErrorContext AKERR_NOIGNORE *(*grid)(struct akbasic_TextSink *self, int *columns, int *rows, int *cellw, int *cellh);
 } akbasic_TextSink;
 
 /** @brief State for the stdio-backed sink. */