Add serial/protocol modules to plan, binary format to arch, -flto to Makefiles

architecture.md: replace JSON control payloads with binary serialization;
add Protocol Serialization section describing little-endian wire format,
put/get buffer layer, and write_*/read_* protocol layer.

planning.md: mark common/media_ctrl/v4l2_ctrl done; insert serial (#4)
and protocol (#6) modules with descriptions.

conventions.md: document -flto and its implication (no manual static for
inlining — compiler handles it at link time).

Makefiles: add -flto to CFLAGS in all four Makefiles.

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

dev/cli/Makefile

@@ -1,6 +1,6 @@
 ROOT   := $(abspath ../..)
 CC      = gcc
-CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -I$(ROOT)/include
+CFLAGS  = -std=c11 -Wall -Wextra -D_GNU_SOURCE -flto -I$(ROOT)/include
 BUILD   = $(ROOT)/build
 
 COMMON_OBJ     = $(BUILD)/common/error.o