Extract shared Makefile config into common.mk

CC, CFLAGS, and BUILD are now defined once in common.mk at the repo root.
Each module and CLI Makefile sets ROOT then includes common.mk, eliminating
the repeated definitions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

common.mk

@@ -0,0 +1,7 @@
+# Included by all module and cli Makefiles.
+# Each including Makefile must set ROOT before including this file:
+#   ROOT := $(abspath <relative path to repo root>)
+
+CC     = gcc
+CFLAGS = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
+BUILD  = $(ROOT)/build