mpdecode_core: slice one shared subs pool per c/v-node array

[LA7LKA]

init_c_v_nodes() called CALLOC() once per c_node and once per v_node for their subs arrays - potentially thousands of small, irregularly-sized allocations per decoded frame, all freed again at the end of the same run_ldpc_decoder() call. On a memory-constrained embedded target (STM32F7, ~16 KB of heap actually available under a real application's other static usage) that churn fragments the heap badly enough to fail a request well before the nominal total bytes are exhausted - confirmed on hardware: a 24-byte allocation failing with a 16200-byte allocation (DecodedBits) having just succeeded moments earlier in the same call. Every CALLOC()/MALLOC() in this file is followed by an unchecked assert(), so a failed allocation is fatal in any build without -DNDEBUG.

Node degree is already fully known before any subs allocation happens, so this sums the total up front and hands out each node's subs as a slice of one shared CALLOC() instead of its own. Same total bytes, same per-node layout and population logic (untouched), just contiguous rather than thousands of separately-managed chunks - fragmentation between nodes is no longer possible. c_nodes[0].subs/v_nodes[0].subs are now the only real allocation base pointers; run_ldpc_decoder()'s cleanup is updated to match (freeing any other node's subs would corrupt the heap, since it's no longer a real malloc()/calloc() return value).

Reproduced and fixed against FreeDV 700D (H_16200_9720) on an STM32F746 running the full application (USB Audio Class, HMI, OLED, etc. all resident) - RX decode now completes cleanly where it previously hung every time on the first real frame.