Add strict pointers: AS PTR TO, POINT ... AT, and the -> operator

A pointer is a distinct declared kind rather than a mode a structure can be in,
which is what "strict" means here: `.` requires a structure on its left and `->`
requires a pointer, neither stands in for the other, and both refusals name the
operator the program should have used. So a reader always knows from the
spelling whether the thing on the left is their own copy or somebody else's
data.

Assignment still copies. POINT is the only way to share, so a program that never
writes it can never be surprised by aliasing -- and `P@ = A@` is refused with a
message saying to POINT it instead, rather than quietly becoming the one
assignment in the language that does not copy.

PTR TO is also the only way a TYPE may refer to itself, since by value it would
have no finite size. That is what makes a linked list possible, and
tests/language/structures/pointers.bas builds one, walks it and renders it.

Rendering follows pointers, so it needs a depth bound where copying does not:
copy stops at a pointer by construction, but two nodes pointing at each other is
easy to write and PRINT would not come back. Four levels, chosen so the bound
bites before the 256-byte render buffer does -- otherwise a cycle would stop
because it ran out of room rather than because it was told to.

Three things the work turned up, all now pinned by tests:

A freshly DIMmed record printed `(UNDEFINED STRING REPRESENTATION FOR 0)` for
every field. Slots now take the type their field declared, so it reads as zeros.

Adding POINT as a verb makes POINT unusable as a type name, and the parser
reported that as "Expected expression or literal" pointing at the line rather
than the problem. The prescan now refuses a reserved word as a type name and
says so.

Field names follow the same reserved-word rule as variable names, enforced by
the loader's own scan -- `TO@` is a bad field name for exactly the reason `TO#`
is a bad variable name. Recorded rather than worked around.

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

include/akbasic/structtype.h

@@ -174,6 +174,15 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_struct_resolve(struct akbasic_Runtime
 /** @brief Evaluate a field access to the value it yields. */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_struct_evaluate_field(struct akbasic_Runtime *obj, akbasic_ASTLeaf *expr, akbasic_Value **dest);
 
+/**
+ * @brief Give every slot of a fresh instance the type its field declared.
+ *
+ * So a record that has only been `DIM`med prints as zeros rather than as
+ * undefined values -- an answer instead of a puzzle. Recurses over the type
+ * graph, which is finite because a `TYPE` cannot contain itself by value.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_struct_init_slots(struct akbasic_Runtime *obj, int typeindex, akbasic_Value *base);
+
 /** @brief Render an instance the way PRINT does, bounded by depth. */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_struct_to_string(struct akbasic_Runtime *obj, int typeindex, akbasic_Value *base, int depth, char *dest, size_t len);
 

include/akbasic/types.h

@@ -54,7 +54,7 @@
  * recursion; it bounds the *rendering* of a structure, where a pointer can make
  * the graph cyclic and PRINT would otherwise not come back.
  */
-#define AKBASIC_MAX_STRUCT_DEPTH	8
+#define AKBASIC_MAX_STRUCT_DEPTH	4
 
 /* Commodore convention: true is -1, not 1. */
 #define AKBASIC_TRUE			-1