Default to the optimized -O3 build; DEBUG=1 for a debug build

Bare 'make -C com' now builds -O3 (optimize.env default flip); the install guides drop OPTIMIZE=1 and document DEBUG=1 for an AddressSanitizer build.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mac/env/optimize.env

@@ -1,16 +1,20 @@
 
-ifdef OPTIMIZE
-# Performance build: make OPTIMIZE=1
-# `override` so a command-line `make OPTIMIZE=1` is remapped to -O3 too;
-# without it, a command-line assignment wins over this `:=` and the literal
-# `1` leaks into CXXFLAGS (g++ then treats `1` as an input file).
-override OPTIMIZE := -O3
-SANITIZE :=
+# Build mode. The DEFAULT is an optimized -O3 build (what installs and releases
+# use). Opt into other modes explicitly:
+#     DEBUG=1     -> debug build: -O0 -g + AddressSanitizer (development)
+#     NOPYTHON=1  -> -O0 -g -DNOPYTHON, no ASan (build without the interpreter)
+# `override` is used throughout so that a stray command-line or environment
+# `OPTIMIZE=...` cannot leak into CXXFLAGS: OPTIMIZE is an internal flag string
+# here, no longer a trigger. (A legacy `make OPTIMIZE=1` still yields -O3, since
+# it falls through to the optimized default below.)
+ifdef DEBUG
+override OPTIMIZE := -O0 -g
+SANITIZE := -fsanitize=address -fno-omit-frame-pointer
 else ifdef NOPYTHON
-OPTIMIZE := -O0 -g -DNOPYTHON
+override OPTIMIZE := -O0 -g -DNOPYTHON
 SANITIZE :=
 else
-# Debug build (default): includes AddressSanitizer
-OPTIMIZE := -O0 -g
-SANITIZE := -fsanitize=address -fno-omit-frame-pointer
+# Performance build (default).
+override OPTIMIZE := -O3
+SANITIZE :=
 endif