Add serial module — little-endian binary serialization primitives

put_u8/16/32/64 and get_u8/16/32/64 (plus signed variants) for packing
and unpacking values at explicit byte offsets in a buffer. Each value is
encoded byte-by-byte with explicit shift operations — no struct casting,
no alignment assumptions, correct on any host endianness.

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

src/modules/serial/Makefile

@@ -0,0 +1,17 @@
+ROOT := $(abspath ../../..)
+include $(ROOT)/common.mk
+
+MODULE_BUILD = $(BUILD)/serial
+
+.PHONY: all clean
+
+all: $(MODULE_BUILD)/serial.o
+
+$(MODULE_BUILD)/serial.o: serial.c $(ROOT)/include/serial.h | $(MODULE_BUILD)
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+$(MODULE_BUILD):
+	mkdir -p $@
+
+clean:
+	rm -f $(MODULE_BUILD)/serial.o

src/modules/serial/serial.c

@@ -0,0 +1,92 @@
+#include <stdint.h>
+#include "serial.h"
+
+/* -- put -- */
+
+void put_u8(uint8_t *buf, uint32_t offset, uint8_t value) {
+	buf[offset] = value;
+}
+
+void put_u16(uint8_t *buf, uint32_t offset, uint16_t value) {
+	buf[offset + 0] = (uint8_t)(value);
+	buf[offset + 1] = (uint8_t)(value >> 8);
+}
+
+void put_u32(uint8_t *buf, uint32_t offset, uint32_t value) {
+	buf[offset + 0] = (uint8_t)(value);
+	buf[offset + 1] = (uint8_t)(value >> 8);
+	buf[offset + 2] = (uint8_t)(value >> 16);
+	buf[offset + 3] = (uint8_t)(value >> 24);
+}
+
+void put_u64(uint8_t *buf, uint32_t offset, uint64_t value) {
+	buf[offset + 0] = (uint8_t)(value);
+	buf[offset + 1] = (uint8_t)(value >> 8);
+	buf[offset + 2] = (uint8_t)(value >> 16);
+	buf[offset + 3] = (uint8_t)(value >> 24);
+	buf[offset + 4] = (uint8_t)(value >> 32);
+	buf[offset + 5] = (uint8_t)(value >> 40);
+	buf[offset + 6] = (uint8_t)(value >> 48);
+	buf[offset + 7] = (uint8_t)(value >> 56);
+}
+
+void put_i8(uint8_t *buf, uint32_t offset, int8_t value) {
+	put_u8(buf, offset, (uint8_t)value);
+}
+
+void put_i16(uint8_t *buf, uint32_t offset, int16_t value) {
+	put_u16(buf, offset, (uint16_t)value);
+}
+
+void put_i32(uint8_t *buf, uint32_t offset, int32_t value) {
+	put_u32(buf, offset, (uint32_t)value);
+}
+
+void put_i64(uint8_t *buf, uint32_t offset, int64_t value) {
+	put_u64(buf, offset, (uint64_t)value);
+}
+
+/* -- get -- */
+
+uint8_t get_u8(const uint8_t *buf, uint32_t offset) {
+	return buf[offset];
+}
+
+uint16_t get_u16(const uint8_t *buf, uint32_t offset) {
+	return (uint16_t)buf[offset + 0]
+	     | ((uint16_t)buf[offset + 1] << 8);
+}
+
+uint32_t get_u32(const uint8_t *buf, uint32_t offset) {
+	return (uint32_t)buf[offset + 0]
+	     | ((uint32_t)buf[offset + 1] << 8)
+	     | ((uint32_t)buf[offset + 2] << 16)
+	     | ((uint32_t)buf[offset + 3] << 24);
+}
+
+uint64_t get_u64(const uint8_t *buf, uint32_t offset) {
+	return (uint64_t)buf[offset + 0]
+	     | ((uint64_t)buf[offset + 1] << 8)
+	     | ((uint64_t)buf[offset + 2] << 16)
+	     | ((uint64_t)buf[offset + 3] << 24)
+	     | ((uint64_t)buf[offset + 4] << 32)
+	     | ((uint64_t)buf[offset + 5] << 40)
+	     | ((uint64_t)buf[offset + 6] << 48)
+	     | ((uint64_t)buf[offset + 7] << 56);
+}
+
+int8_t get_i8(const uint8_t *buf, uint32_t offset) {
+	return (int8_t)get_u8(buf, offset);
+}
+
+int16_t get_i16(const uint8_t *buf, uint32_t offset) {
+	return (int16_t)get_u16(buf, offset);
+}
+
+int32_t get_i32(const uint8_t *buf, uint32_t offset) {
+	return (int32_t)get_u32(buf, offset);
+}
+
+int64_t get_i64(const uint8_t *buf, uint32_t offset) {
+	return (int64_t)get_u64(buf, offset);
+}