Document structures: a chapter, the architecture, and the differences

docs/16-structures.md is the feature: records, nesting, copy-on-assign, strict
pointers, lists, what is checked and what is not, and how a host shares its own
C structs. Every example in it is executed by docs_examples and byte-compared,
including the refusals -- so a message that changes fails the suite rather than
quietly making the chapter wrong.

The chapter makes one contrast explicitly, because it is the question a reader
will actually have: a misspelled *field* is refused and a misspelled *variable*
still prints zero. The rule underneath is that what the program declared gets
checked and what it did not gets shrugged at -- a variable's name is never
declared, a TYPE's field list is. Structures end up the strictest thing in the
language, not from a higher standard but because they are the only named thing
whose valid spellings are written down.

Chapter 14 gains the layout: an instance is a contiguous run of value slots with
a diagram of where the fields sit, the three-pass prescan and why each pass
exists, why the copy cannot live in akbasic_value_clone(), and why the render
depth bound is four rather than eight. Chapter 3 gains the @ suffix, chapter 13
records that all of this is an addition BASIC 7.0 has nothing like, and the verb
reference gains TYPE, POINT and DIM ... AS.

MAINTENANCE.md gains the two rules that are on a maintainer rather than on a
test: a structure copy must not go through clone, and a field chain gets its own
leaf field. TODO.md section 5 records what was invented and the three limits
that are ours, and section 8 records the two defects the work exposed.

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

MAINTENANCE.md

@@ -497,6 +497,27 @@ So: still do it during single-threaded init, before the host game spawns anythin
 because the call would race, but because there is one operation in the registry that cannot be
 made safe and this is the discipline that avoids needing it.
 
+### Structures reuse the array machinery, deliberately
+
+`TYPE` declares a record, so an instance has a known slot count and takes one contiguous
+run from the same value pool `DIM A#(10)` draws from. **There is no structure pool**, and
+adding one would be the wrong instinct: the only new table holds *descriptors* — names and
+slot offsets — and the data goes where array data already goes.
+
+Two rules for changing any of it:
+
+- **A structure copy must not go through `akbasic_value_clone()`.** Clone copies one slot,
+  and one slot holds a *reference* to an instance rather than the instance, so a structure
+  taking that path aliases instead of copying — which is the semantics the language
+  deliberately does not have. `akbasic_environment_assign()` intercepts first and calls
+  `akbasic_struct_copy()`. If you add a place a structure can be assigned, it goes through
+  that, not through clone.
+- **A field chain gets its own leaf type and its own link field.** `include/akbasic/grammar.h`
+  records three separate defects that came from giving one link field two meanings;
+  `AKBASIC_LEAF_FIELD` keeps its base on `.left`, which nothing else on that leaf type uses.
+
+`docs/14-architecture.md` has the layout diagram and the three-pass prescan.
+
 ### Nothing calls malloc
 
 `libakgl`'s hard rule, and ours: obtain objects from `akgl_heap_next_*` and release them back,