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>

TODO.md

@@ -1960,8 +1960,23 @@ in either corpus runs for minutes, which is why the first of these had never bee
     calling `WINDOW` "ordinary work here rather than anything blocked", which is now half true:
     the work was done and then not plumbed.
 
-    Worth deciding at the same time: **a program still has no way to ask how big the text grid
-    is.** `RGR(1)` and `RGR(2)` give the window in pixels, and nothing gives columns, rows or
+    ~~Worth deciding at the same time: **a program still has no way to ask how big the text
+    grid is.**~~ **Done, and with BASIC 7.0's own function rather than more `RGR` fields.**
+    `RWINDOW(0)` is the current text window's rows and `RWINDOW(1)` its columns -- the C128
+    has exactly this function and this interpreter had never implemented it. `RWINDOW(2)` is
+    refused by name: it reports a 40 or 80 column screen mode and there is no such mode here,
+    so answering 0 would be a plausible lie.
+
+    The cell size in pixels is `RGR(3)` and `RGR(4)`, beside the surface's own dimensions,
+    because that is a fact about the surface -- and because `RWINDOW` reports the *window*,
+    which makes dividing `RGR(1)` by a column count wrong the moment a program calls
+    `WINDOW`. Both read a new optional `grid` entry point on `akbasic_TextSink`, implemented
+    by the akgl sink and forwarded by the tee, NULL everywhere else so both refuse by name.
+
+    `RGR(3)` on the standalone build now answers 16 -- the number the Breakout listing had
+    measured by hand and written out as a constant.
+
+    The original note: `RGR(1)` and `RGR(2)` give the window in pixels, and nothing gives columns, rows or
     the cell size, so anything that wants to place a character *and* a sprite at the same spot
     has to hardcode a cell size measured against the bundled font -- which is what the Breakout
     in `examples/breakout/characters` does, and it is the one thing in that listing that will break