Honour a subscript in SSHAPE and GSHAPE

`shape_variable()` took the identifier off the leaf and looked the variable up
without ever evaluating the subscript, and both verbs then addressed element
zero with a literal. So `SSHAPE SH$(2), ...` wrote the handle into `SH$(0)` and
`GSHAPE SH$(2)` stamped whatever was in `SH$(0)`.

Ordinary assignment and `PRINT` honour the subscript, which is what made this
expensive: a program keeping several saved shapes in an array got every one of
them resolving to the same element, silently, and the only symptom was that
every stamp came out as the last shape captured. The Breakout in examples/ keeps
its six brick stamps in six separate scalars for exactly this reason.

`SPRSAV` was the counter-example and is the model -- it evaluates its argument
and handles an array element correctly. The subscript resolution itself is now
shared: `collect_subscripts()` comes out of src/environment.c as
`akbasic_environment_collect_subscripts()`, so a verb taking a variable by name
resolves a subscript the same way assignment does rather than each verb deciding
for itself.

tests/graphics_verbs.c covers TODO.md's reduction -- which used to print
"[SHAPE:0] []" and now prints "[] [SHAPE:0]" -- and the case a program actually
wants: two shapes captured into two elements, each stamped back through its own,
asserted against the device log so a fix that merely made the strings look right
would not pass.

Chapter 18's trap 4 becomes history, and Chapter 6 says an array works.

TODO.md section 9 item 6, struck.

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

docs/06-graphics.md

@@ -227,6 +227,9 @@ parallel passes; see Chapter 13.
 
 One disc drawn, captured, and stamped three times.
 
+A string array works: `SSHAPE SH$(2), ...` writes into element 2 and `GSHAPE SH$(2)`
+stamps what element 2 names, so a program can keep a set of shapes in one array.
+
 **`A$` holds a handle, not the pixels.** On a C128 the string holds the bitmap, so a
 program could save it to disk or take its `LEN`. Here a string is a fixed 255 bytes and
 the region is a device surface, so what goes in the string is a reference to it —