feat: Add Selective Source Routing (SSR) support to c-mesh-api

Implement SSR (Selective Source Routing) integration for the Linux gateway,
enabling automatic source-routed packet transmission based on first-hop
information learned from mesh node registrations.

Key additions:

- First-Hop Table (FHT): Open-addressing hash table with linear probing,
  tombstone deletion, and automatic growth. Maintains routes with
  configurable validity windows and automatic expiry purging.

- SSR module: Integration layer that maintains the FHT, processes incoming
  SSR registration indications from the sink, and provides route lookup
  for the send path.

- DSAP extensions: New dsap_data_tx_ssr_request() function for sending
  single-frame and fragmented packets with source-routing hop lists.
  Refactored common TX logic into dsap_data_tx_request_internal().

- WPC integration: Automatic SSR route lookup in WPC_send_data_with_options().
  Falls back to traditional routing if no SSR route is available.
  SSR module initialized/deinitialized with WPC lifecycle.

- MSAP handler: New msap_ssr_registration_indication_handler() processes
  incoming registration packets and updates the FHT.

- Platform integration: SSR purge called periodically from the dispatch
  thread alongside reassembly garbage collection.

- Comprehensive unit tests covering FHT operations, SSR module lifecycle,
  frame layout validation, and integration with WPC send path.

Author: mortewm

lib/api/wpc.h

@@ -952,6 +952,8 @@ app_res_e WPC_send_data(const uint8_t * bytes,
  * \param   message_p
  *          The message to send
  * \return  Return code of the operation
+ * \note    If a valid SSR first-hop is available for the destination and the
+ *          payload can be routed through SSR, SSR routing is used automatically.
  */
 app_res_e WPC_send_data_with_options(const app_message_t * message_p);
 

lib/platform/linux/platform.c

@@ -16,6 +16,7 @@
 #include "logger.h"
 #include "platform.h"
 #include "reassembly.h"
+#include "ssr.h"
 #include "wpc_proto.h"
 
 // Maximum number of indication to be retrieved from a single poll
@@ -112,6 +113,7 @@ static void * dispatch_indication(void * unused)
 
             // Force a garbage collect (to be sure it's called even if no frag are received)
             reassembly_garbage_collect();
+            ssr_purge_expired();
 
             // Check if we wake up but nothing in queue
             if (m_queue_empty)

lib/wpc/CMakeLists.txt

@@ -7,6 +7,8 @@ add_library(wpc STATIC
     ${CMAKE_CURRENT_LIST_DIR}/wpc.c
     ${CMAKE_CURRENT_LIST_DIR}/wpc_internal.c
     ${CMAKE_CURRENT_LIST_DIR}/reassembly/reassembly.c
+    ${CMAKE_CURRENT_LIST_DIR}/ssr/fht.c
+    ${CMAKE_CURRENT_LIST_DIR}/ssr/ssr.c
 )
 
 target_include_directories(wpc PUBLIC