Phase 1: expression calculator with direct mode

GW-BASIC reimplementation in C11, using Microsoft's open-sourced 8088
assembly as the authoritative reference.

Tokenizer (CRUNCH/LIST), expression evaluator with operator precedence,
all math functions (SIN, COS, TAN, ATN, SQR, LOG, EXP, RND, etc.),
string functions (LEFT$, RIGHT$, MID$, CHR$, ASC, VAL, STR$, etc.),
PRINT statement with comma/semicolon zones, TAB(), SPC().

Handles integer/single/double types with correct promotion, D exponent
for double-precision literals, type suffixes (%, !, #), &H hex/&O octal
literals, MBF conversion routines, and GW-BASIC-compatible number
formatting.

Platform-independent via HAL vtable; POSIX backend included.

tests/programs/string_ops.bas

@@ -0,0 +1,14 @@
+PRINT LEFT$("HELLO WORLD", 5)
+PRINT RIGHT$("HELLO WORLD", 5)
+PRINT MID$("HELLO WORLD", 7, 5)
+PRINT LEN("TEST")
+PRINT CHR$(65)
+PRINT ASC("A")
+PRINT VAL("3.14")
+PRINT STR$(42)
+PRINT SPACE$(5)+"X"
+PRINT STRING$(10, 42)
+PRINT HEX$(255)
+PRINT OCT$(255)
+PRINT INSTR("HELLO WORLD", "WORLD")
+PRINT "ABC" + "DEF"