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/math_ops.bas

@@ -0,0 +1,19 @@
+PRINT "Arithmetic:"
+PRINT 2+2
+PRINT 10-3
+PRINT 6*7
+PRINT 100/3
+PRINT 2^10
+PRINT 17 MOD 5
+PRINT 17\5
+PRINT "Functions:"
+PRINT SIN(3.14159/2)
+PRINT COS(0)
+PRINT SQR(144)
+PRINT ABS(-42)
+PRINT INT(3.7)
+PRINT FIX(-3.7)
+PRINT LOG(1)
+PRINT EXP(0)
+PRINT SGN(-5)
+PRINT ATN(1)*4