Implement generators: GEN, EMIT, END GEN, FOR EACH and DO EACH (issue 57)

Adds generator support per the plan in issue 57:

- environment.h: isGenerator/generatorFn on a GEN call's own environment,
  isEachLoop and forGeneratorEnv on a FOR EACH/DO EACH loop's own
  environment.
- runtime.c: splits akbasic_runtime_prev_environment() into
  akbasic_runtime_detach_environment() (return to parent without releasing)
  and akbasic_runtime_release_environment() (give variables and the pool
  slot back, on any environment); prev_environment() is now the two in
  sequence. akbasic_runtime_call_function() refuses to call a GEN like an
  ordinary function.
- verbs.c/verbs.h/scanner: new keywords GEN, EMIT, EACH, IN and the compound
  verb "END GEN" (built by a new akbasic_parse_end(), the same trick
  akbasic_parse_print() uses for PRINT #).
- parser_commands.c: akbasic_parse_gen() (modelled on multi-line DEF),
  akbasic_parse_end(), and EACH branches in akbasic_parse_for()/
  akbasic_parse_do().
- runtime_generator.c (new): akbasic_cmd_gen, akbasic_cmd_emit,
  akbasic_cmd_end_gen, and the invoke/pump/release machinery FOR EACH, DO
  EACH, NEXT and LOOP share. EMIT walks up to the nearest isGenerator
  ancestor rather than assuming it is standing directly in the GEN's own
  call frame, because a GEN body may nest its own FOR/DO/GOSUB around an
  EMIT -- the issue's own ROOMOBJECTS example does exactly that.
- runtime_commands.c/runtime_structure.c: EACH branches in cmd_for/cmd_do,
  matching EACH branches in cmd_next/cmd_loop, and forGeneratorEnv release
  on every path that can abandon a live generator (EXIT, a NEXT that pops
  for a mismatched loop variable).

Deviates from the plan in one place: FunctionDef gained an isGenerator flag
(not in the plan's field list) because refusing a GEN called like a
function has to happen before anything is pushed. Relying on EMIT's own
isGenerator check for that case doesn't work: akbasic_runtime_call_function()
drives its own step loop the same way akbasic_runtime_pump_generator() does,
and a BASIC-level error inside that loop is swallowed by process_line_run()
as reported-but-not-propagated, so the call would silently "succeed" with a
meaningless return value instead of failing.

Also: a zero-argument parameter list is not supported by the DEF/GEN
parameter parser this reuses (a pre-existing limitation, not
generator-specific); every generator in the tests takes at least one
parameter as a result.

Tests: tests/generators.c (pool exhaustion under repeated EXIT, calling a
GEN like a function, EMIT outside a GEN, self-recursion, sibling/nested
invocations) and tests/language/flowcontrol/generators_*.bas -- the
issue's own ROOMOBJECTS example in both loop shapes, an empty generator,
non-numeric EMIT, nested/interleaved invocations, and three error-path
golden cases. Docs: control-flow chapter 4 gets a GEN/EMIT/FOR EACH/DO EACH
section, the verb reference gets GEN/EMIT/END GEN entries and updated
FOR/DO/NEXT/LOOP/EXIT rows, and architecture chapter 14 documents the
detach/release split and the two-environment generator invocation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/11-verb-reference.md

@@ -37,17 +37,20 @@ for the reasoning in each case.
 | `DIM` … `AS` | `DIM S@ AS T`, `DIM P@ AS PTR TO T` | Make a structure, or a strict pointer to one. See Chapter 16. |
 | `DIRECTORY` | `DIRECTORY` | **Refused.** Not written yet; the standard-library wrapper it waited on has landed. |
 | `DLOAD` | `DLOAD "name"` | Load a program from a file. |
-| `DO` | `DO [WHILE c | UNTIL c]` | Start a loop. The condition may be here, on the `LOOP`, or neither. |
+| `DO` | `DO [WHILE c | UNTIL c]`, `DO EACH V IN gen(args)` | Start a loop. The condition may be here, on the `LOOP`, or neither. `EACH` consumes a `GEN` instead; see Chapter 4. |
 | `DOPEN` | `DOPEN n, "name" [,W]` | Open a file on channel `n`. `W` opens it for writing. |
 | `DRAW` | `DRAW src, x, y [TO x, y ...]` | Plot a point or draw a polyline. |
 | `DSAVE` | `DSAVE "name"` | Save the program to a file. |
 | `DVERIFY` | `DVERIFY "name"` | The other name for `VERIFY`. |
+| `EMIT` | `EMIT expr` | Yield one value from a `GEN` body. Only valid inside one; see Chapter 4. |
 | `END` | `END` | Stop the program. Does not arm `CONT`. |
+| `END GEN` | `END GEN` | Close a `GEN` body, the way `RETURN` closes a multi-line `DEF`. |
 | `ENVELOPE` | `ENVELOPE n, a, d, s, r` | Define one of `PLAY`'s ten envelope presets. |
-| `EXIT` | `EXIT` | Leave the innermost `FOR` or `DO` loop. |
+| `EXIT` | `EXIT` | Leave the innermost `FOR`, `FOR EACH`, `DO` or `DO EACH` loop. |
 | `FETCH` | `FETCH count, from, to` | Copy bytes. The same as `STASH`; there is no expansion RAM. |
 | `FILTER` | `FILTER ...` | **Refused.** There is no filter stage in the audio backend. |
-| `FOR` | `FOR V = a TO b [STEP c]` | Start a counted loop, ended by `NEXT`. |
+| `FOR` | `FOR V = a TO b [STEP c]`, `FOR EACH V IN gen(args)` | Start a counted loop, ended by `NEXT`. `EACH` consumes a `GEN` instead; see Chapter 4. |
+| `GEN` | `GEN NAME(args) ... END GEN` | Define a generator: a subroutine that yields more than once via `EMIT`, consumed by `FOR EACH`/`DO EACH`. See Chapter 4. |
 | `GET` | `GET V` | Take a keystroke if one is waiting, without stopping. |
 | `GETKEY` | `GETKEY V` | Wait for a keystroke, holding the program but not the host. |
 | `GETMENU` | `GETMENU n, V%` | Wait for a menu choice, holding the program but not the host. Assigns the entry number. See Chapter 19. |
@@ -67,11 +70,11 @@ for the reasoning in each case.
 | `LIST` | `LIST [n][-n]` | List the program, or part of it. |
 | `LOAD` | `LOAD "name"` | The other name for `DLOAD`. |
 | `LOCATE` | `LOCATE x, y` | Move the pixel cursor. |
-| `LOOP` | `LOOP [WHILE c | UNTIL c]` | End a `DO` loop. |
+| `LOOP` | `LOOP [WHILE c | UNTIL c]` | End a `DO` loop, including a `DO EACH`. |
 | `MENU` | `MENU [n [,"item", ...]]` | Show a menu the player picks from. No entries retires it; no arguments retire them all. See Chapter 19. |
 | `MOVSPR` | `MOVSPR n, ...` | Move a sprite. Four forms; see Chapter 8. |
 | `NEW` | `NEW` | Erase the program and every variable. |
-| `NEXT` | `NEXT V` | End a `FOR` loop and advance its counter. |
+| `NEXT` | `NEXT V` | End a `FOR` loop and advance its counter, or resume a `FOR EACH` for its next value. |
 | `ON` | `ON e GOTO|GOSUB t [,...]` | Branch to the `e`th target, counting from one. |
 | `PAINT` | `PAINT src, x, y` | Flood-fill the region containing a point. |
 | `PLAY` | `PLAY "notes"` | Queue notes. Does not block. |