Add --warn and --safe memory safety flags to the compiler
Three progressive levels for gwbasic-compile:
--warn: static analysis warnings (uninitialized variables, GOTO to
nonexistent line, unreachable code detection). Zero runtime cost.
--safe (implies --warn): runtime checked integer arithmetic via
gw_int_add/sub/mul/neg matching real GW-BASIC overflow semantics,
enhanced array bounds diagnostics with variable names and line numbers,
GOSUB stack overflow diagnostics with source line reporting.
--safe=sanitize (implies --safe): passes -fsanitize=address,undefined
to gcc for full memory error detection.
Also: fix pre-existing missing closing paren in array LET-to-integer
codegen, add strpool_pin/unpin infrastructure, add compiler optimization
flags and memory safety sections to roadmap.
72/72 interpreter tests pass. 64/64 eligible compiler tests pass in
--safe mode.
20ecdae938