Unbreak the three example programs the RND merge left behind

Adding native RND and ASC (ae2c702) made RND a function name, and a suffixed
identifier that collides with one is refused -- "SYNTAX ERROR Reserved word in
variable name". Three example programs held their PRNG output in a variable
called RND#, or a host field called RND%, and none of them had run since:

  - examples/galaga/ bound RND% as a host field on both ENEMY and GAME. The
    BASIC-visible name is ROLL% now; the C member stays `rnd`. This one was
    caught by example_galaga and example_galaga_interop, which have been
    failing.
  - examples/breakout/characters/breakout.bas and examples/megademo/
    megademo.bas both use RND# for their LCG output, renamed to ROLL#. Neither
    is in any test, so neither failure was visible.

examples/breakout/sprites/breakout.bas was broken a second way: seven REM lines
the reader refuses. Worth recording that the ceiling is not the one the message
names -- src/sink_stdio.c fails when the read filled the buffer without seeing
a terminator, so with AKBASIC_MAX_LINE_LENGTH at 80 the message says "79
character limit" and the real maximum is 78, because a 79-character line leaves
no room for the newline. The sweeps that fixed the corpus and the megademo for
this did not reach this file. The seven comments are reflowed.

The prose went stale with the code. Chapter 21 said "there is no RND verb in
this dialect; issue #16 tracks adding one", chapter 17's historical aside
offered an LCG that no longer parses, and four REM blocks across the two games
said the same thing. All of them now say RND exists, and say why these programs
keep their own generator anyway: the sequence has to be reproducible for a
headless run to be the same game on every machine, which is what lets
interop_test.c assert exact counts.

Chapter 21 also gains the rule that bit them, since a reader writing a host
type will hit it: a host field name is a bare word and shares a namespace with
every verb and function.

None of this came from the submodule bump -- all three were already broken on
main. It was found by running the tutorial games, which nothing else does;
that gap is akbasic issue #58.

Verified: all three run clean under the dummy drivers, and 114/114 default,
116/116 with akgl.

Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>
Co-Authored-By: Claude Code (Claude Opus 5, claude-opus-5[1m]) <noreply@anthropic.com>

examples/galaga/galaga.bas

@@ -50,7 +50,7 @@ DEF DECIDEFIRE(DT%)
   DX% = GAME@.PLAYERX% - ACTOR@.X%
   IF ABS(DX%) > 140 THEN RETURN 0
   IF ACTOR@.Y% > GAME@.PLAYERY% THEN RETURN 0
-  IF SELF@.RND% < DT% * 1.5 THEN SELF@.FIRE# = 1
+  IF SELF@.ROLL% < DT% * 1.5 THEN SELF@.FIRE# = 1
   RETURN 0
 
 REM Bee: enter, breathe in formation, occasionally dive nearly straight.
@@ -65,7 +65,7 @@ DEF UPDATEBEE(DT%)
   IF (S# AND 2) > 0 THEN BEGIN
     ACTOR@.X% = SELF@.HOMEX% + SIN(SELF@.T% * 1.7) * 16
     ACTOR@.Y% = SELF@.HOMEY%
-    IF SELF@.RND% < DT% * 0.04 THEN SELF@.STATE# = 4 : SELF@.T% = 0
+    IF SELF@.ROLL% < DT% * 0.04 THEN SELF@.STATE# = 4 : SELF@.T% = 0
   BEND
   IF (S# AND 4) > 0 THEN BEGIN
     R# = DIVESTEP(DT%, 130, 0.2)
@@ -85,7 +85,7 @@ DEF UPDATEBFLY(DT%)
   IF (S# AND 2) > 0 THEN BEGIN
     ACTOR@.X% = SELF@.HOMEX% + SIN(SELF@.T% * 2.1) * 24
     ACTOR@.Y% = SELF@.HOMEY%
-    IF SELF@.RND% < DT% * 0.05 THEN SELF@.STATE# = 4 : SELF@.T% = 0
+    IF SELF@.ROLL% < DT% * 0.05 THEN SELF@.STATE# = 4 : SELF@.T% = 0
   BEND
   IF (S# AND 4) > 0 THEN BEGIN
     R# = DIVESTEP(DT%, 260, 0.1)
@@ -108,7 +108,7 @@ DEF UPDATEBOSS(DT%)
   IF (S# AND 2) > 0 THEN BEGIN
     ACTOR@.X% = SELF@.HOMEX% + SIN(SELF@.T% * 1.1) * 10
     ACTOR@.Y% = SELF@.HOMEY%
-    IF SELF@.RND% < DT% * 0.03 THEN SELF@.STATE# = 4 : SELF@.T% = 0
+    IF SELF@.ROLL% < DT% * 0.03 THEN SELF@.STATE# = 4 : SELF@.T% = 0
   BEND
   IF (S# AND 4) > 0 THEN BEGIN
     R# = DIVESTEP(DT%, 60, 0.9)