feat: backtick raw/multi-line strings + QBE backend fixes

- Add backtick raw string literals (Go-style ): no escape processing, multi-line
- C backend: handle backtick strings in emitExpr
- Self-hosted lexer: lexScanBacktickString support
- QBE backend: handle backtick strings in data sections
- QBE: remove broken extern declarations (QBE resolves at link time)
- QBE: fix comparison result types (ceql → w, extend to l)
- QBE: fix SSA parameter name clash (_p_ prefix for params)
- QBE: const inlining via HirLower + QBE emitter
- QBE: proper type mapping for bool/char types
- hir_lower: collect dkConst declarations for const propagation

stdlib/runtime.c

@@ -479,6 +479,7 @@ char* bux_read_file(const char* path) {
     long size = ftell(f);
     fseek(f, 0, SEEK_SET);
     char* buf = (char*)bux_alloc((size_t)size + 1);
+    if (!buf) { fclose(f); return NULL; }
     size_t read = fread(buf, 1, (size_t)size, f);
     fclose(f);
     buf[read] = '\0';