Refactor platform ports for bare-metal support

[DemonGiggle]

Summary

• split shared runtime sources into src/core and platform-specific ports into src/platform/{posix,freertos,baremetal}
• add EC_PLATFORM=BAREMETAL with UART HAL, socket HAL, and direct-MMIO support
• preserve the FreeRTOS task entry point in the FreeRTOS platform port
• update README/spec/plan docs and add a bare-metal CI build matrix entry

Verification

• cmake -S . -B build-posix -DEC_PLATFORM=POSIX -DEC_ENABLE_TLS=OFF
• cmake --build build-posix
• ctest --test-dir build-posix --output-on-failure
• cmake -S . -B build-host-sim -DEC_PLATFORM=POSIX -DEC_HOST_SIM=ON -DEC_ENABLE_TLS=OFF
• cmake --build build-host-sim
• ctest --test-dir build-host-sim --output-on-failure
• cmake -S . -B build-baremetal -DEC_PLATFORM=BAREMETAL -DEC_ENABLE_TLS=OFF
• cmake --build build-baremetal
• cmake -S . -B build-baremetal-tls -DEC_PLATFORM=BAREMETAL -DEC_ENABLE_TLS=ON
• cmake --build build-baremetal-tls

[gemini-code-assist]

Code Review

This pull request refactors the project to support bare-metal targets by separating core logic from platform-specific code and introducing HAL bridges for UART and sockets. Feedback focuses on optimizing performance by caching socket options, preventing CPU starvation in FreeRTOS tasks, and improving code safety and style through defensive pointer checks and consistent use of NULL.

[gemini-code-assist]

The task enters a busy-wait loop if the agent loop exits. In a FreeRTOS environment, this will starve lower-priority tasks from executing if this task has a higher priority. It is recommended to delete the task or use a blocking delay to yield the CPU.

Suggested change

	for (;;) {}
	vTaskDelete(NULL);

[gemini-code-assist]

Calling setsockopt on every recv is inefficient as it involves a system call. Since the timeout is often constant for a session, consider caching the current timeout value in the ec_socket structure and only invoking setsockopt if the requested timeout_ms has changed.

[gemini-code-assist]

Since this bare-metal port uses a static singleton s_socket, it is safer to verify that the provided sock pointer actually matches the address of the singleton. This provides a defensive guard against callers passing invalid or uninitialized pointers.

Suggested change

	if (!sock || !sock->in_use || !s_hal || !s_hal->send) {
	if (sock != &s_socket || !sock->in_use || !s_hal || !s_hal->send) {

[gemini-code-assist]

Use NULL instead of 0 for pointer initialization to improve code clarity and follow standard C idioms for pointer types.

Suggested change

	static const ec_io_uart_hal_t *s_uart_hal = 0;
	static const ec_io_uart_hal_t *s_uart_hal = NULL;

[gemini-code-assist]

Use NULL instead of 0 for pointer initialization to adhere to standard C style conventions.

Suggested change

	static const ec_socket_baremetal_hal_t *s_hal = 0;
	static const ec_socket_baremetal_hal_t *s_hal = NULL;

[gemini-code-assist]

The function returns a pointer type (ec_socket_t *), so it should return NULL instead of 0 for consistency and readability.

Suggested change

	return 0;
	return NULL;