fix(v4.0.0): shorter chains of calls in the compiler capsule

Measured, compiling a DO loop left one return entry spare under
INTERPRET and a defining word called from inside another word
overflowed the nine-entry return stack (D-2); the prompt loop will take
one more.

C@ and C! shift without a loop.  The code generator shifts each opcode
into the word being built, so nothing is placed by a counted shift, and
the address masks come from a table.  The dictionary search, the
in-liner, the end of a loop and the making of a data word are each one
word reached by a jump.

Every kind of line now leaves at least three return entries spare while
it compiles; a word run from the interpreter has six; and CREATE ...
DOES> works from the prompt, from a word, and from a word that calls
that.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/tests/host_map.h

@@ -33,8 +33,8 @@
 /* each capsule file's scratch cells */
 #define PVARS        (TOP - 24)                 /* (P): input.v4, 8 cells */
 #define DVARS        (TOP - 32)                 /* (D): dict.v4, 5 cells */
-#define CGVARS       (TOP - 48)                 /* (CG): codegen.v4, 13 cells */
-#define CVARS        (TOP - 58)                 /* (C): compile.v4, 8 cells */
+#define CGVARS       (TOP - 48)                 /* (CG): codegen.v4, 14 cells */
+#define CVARS        (TOP - 62)                 /* (C): compile.v4, 10 cells */
 
 /* buffers (word addresses; a byte address is four times this) */
 #define CFS_W        (TOP - 96)                 /* the control-flow stack, 32 cells */