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>

conventions.md

@@ -4,6 +4,7 @@
 
 - **C11** throughout (`-std=c11`)
 - **`_GNU_SOURCE`** defined via `-D_GNU_SOURCE` in all Makefiles — enables the full GNU/Linux feature set (POSIX 2008, BSD, GNU and Linux-specific extensions); do not define this in source files
+- **LTO enabled** via `-flto` in all Makefiles — the compiler handles inlining across translation units at link time; do not use `static` on functions solely to enable inlining
 - Target platform: Linux (V4L2, epoll, etc. are Linux-specific)
 
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