Fix crash: allocate heap memory for nodes added to rapid table

[AlanRuno]

Summary

Fixes a "free(): invalid pointer" crash that occurred when rebuilding the rapid table after receiving the genesis block.

The root cause was that a stack-allocated node_stake variable was being passed directly to mxd_add_to_rapid_table(). The rapid table takes ownership of node pointers and frees them later (e.g., in mxd_rebuild_rapid_table_from_blockchain). When the rapid table tried to free a stack pointer, it crashed.

The fix allocates heap memory for the node before adding it to the rapid table, and frees it if the add operation fails.

Review & Testing Checklist for Human

• Verify ownership model: Confirm that mxd_add_to_rapid_table() takes ownership of the pointer (stores it directly, doesn't copy). If it copies the data, this change would cause a memory leak.
• Check other call sites: Search for other places that call mxd_add_to_rapid_table() to ensure they all pass heap-allocated pointers. Run: grep -rn "mxd_add_to_rapid_table" src/
• Run 10-node test: Deploy the fix and run the MXDTestSuite 10-node test to verify nodes no longer crash when receiving the genesis block and all reach Height: 1.

Notes

Crash log that led to this fix:

[2026-01-11T07:17:56Z] INFO [sync]: Stored unsolicited block at height 0
[2026-01-11T07:17:57Z] INFO [rsc]: Rebuilding rapid table from genesis block
free(): invalid pointer

Link to Devin run: https://app.devin.ai/sessions/f0a6357d898e440ea3745d77dfb91d8d
Requested by: Runo (runonetworks@gmail.com) / @AlanRuno

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