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/04-control-flow.md

@@ -146,6 +146,89 @@ condition at all loops forever until an `EXIT` or a `GOTO` leaves it.
 
 `EXIT` works here too.
 
+## GEN ... EMIT and FOR EACH / DO EACH
+
+A `GEN` is a subroutine that hands back more than one value, one at a time, instead of
+returning once. It looks like a multi-line `DEF`, except its body runs `EMIT` where a
+function would `RETURN`, and it ends in `END GEN` rather than `RETURN`:
+
+```basic
+10 GEN COUNTUP(N#)
+20     FOR I# = 1 TO N#
+30         EMIT I#
+40     NEXT I#
+50 END GEN
+60 FOR EACH V# IN COUNTUP(3)
+70     PRINT V#
+80 NEXT V#
+```
+
+```output
+1
+2
+3
+```
+
+`FOR EACH` is what runs a `GEN`: it calls `COUNTUP(3)` the way `FOR EACH V# IN` says,
+and each `EMIT` inside the generator's body becomes one trip through the loop, with `V#`
+holding whatever was emitted. When the generator's body reaches `END GEN` with nothing
+left to emit, the loop ends -- there is no separate "no more values" check to write.
+
+`DO EACH ... LOOP` does the same thing:
+
+```basic
+10 GEN COUNTUP(N#)
+20     FOR I# = 1 TO N#
+30         EMIT I#
+40     NEXT I#
+50 END GEN
+60 DO EACH V# IN COUNTUP(3)
+70     PRINT V#
+80 LOOP
+```
+
+```output
+1
+2
+3
+```
+
+A `GEN`'s body is ordinary BASIC: it may hold its own `FOR`, `DO`, `IF` or `GOSUB`
+around the `EMIT`s, and even invoke another `GEN` with its own `FOR EACH`/`DO EACH` --
+the same generator invoked with different arguments, nested or side by side, is not
+recursion. A `GEN` invoking *itself* from within its own currently-running body is
+refused, the same way it would be an error to call a function that has not returned yet
+by name from inside its own body without meaning to recurse.
+
+`EXIT` leaves a `FOR EACH`/`DO EACH` loop early, exactly as it does a plain `FOR` or
+`DO`, and the generator it was consuming stops there -- nothing forces the rest of it to
+run just because the loop started it:
+
+```basic
+10 GEN COUNTUP(N#)
+20     FOR I# = 1 TO N#
+30         EMIT I#
+40     NEXT I#
+50 END GEN
+60 FOR EACH V# IN COUNTUP(100)
+70     PRINT V#
+80     IF V# = 3 THEN EXIT
+90 NEXT V#
+100 PRINT "STOPPED"
+```
+
+```output
+1
+2
+3
+STOPPED
+```
+
+A `GEN` shares its namespace with `DEF` -- the same name cannot be both -- but it is not
+a function and cannot be called like one: `X# = COUNTUP(3)` is refused, because nothing
+about an ordinary call means "resume where the last `EMIT` left off." `FOR EACH`/`DO
+EACH` is the only thing that consumes a `GEN`.
+
 ## GOTO and GOSUB
 
 ```basic