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.

include/hal.h

@@ -0,0 +1,34 @@
+#ifndef GW_HAL_H
+#define GW_HAL_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+/* Hardware Abstraction Layer vtable */
+typedef struct hal_ops {
+    /* Terminal I/O */
+    void (*putch)(int ch);
+    void (*puts)(const char *s);
+    int  (*getch)(void);           /* blocking read */
+    bool (*kbhit)(void);           /* key available? */
+    void (*locate)(int row, int col);
+    int  (*get_cursor_row)(void);
+    int  (*get_cursor_col)(void);
+    void (*cls)(void);
+    void (*set_width)(int cols);
+
+    /* Terminal properties */
+    int  screen_width;
+    int  screen_height;
+
+    /* Lifecycle */
+    void (*init)(void);
+    void (*shutdown)(void);
+} hal_ops_t;
+
+extern hal_ops_t *gw_hal;
+
+/* Platform implementations */
+hal_ops_t *hal_posix_create(void);
+
+#endif