fix(node): correct init error paths (double free / double thread_pool deinit)

[ch4r10t33r]

Summary

• BeamNode.init (pkgs/node/src/node.zig): Removed the overlapping errdefer allocator.destroy(chain) that ran after the errdefer { chain.deinit(); allocator.destroy(chain); } on any error after BeamChain.init succeeded. Unwind then called destroy on the same pointer twice → DebugAllocator double free (seen when later init steps fail, e.g. metrics bind AddressInUse / ServerStartupFailed).
• Node.init (pkgs/cli/src/node.zig): On xmss.setupVerifier() failure, dropped the extra thread_pool.deinit() — errdefer self.thread_pool.deinit() already runs, so the manual call was a double deinit.

Context

Operators hit error.AddressInUse on the metrics port; startMetricsServer returns ServerStartupFailed and Node.init unwinds. The duplicate *BeamChain teardown amplified that into allocator corruption (double free + bogus leak reports).

Testing

• zig build (full default pipeline) — succeeded locally.

[zclawz]

Reviewed PR #877. No blocking issues found.

What I checked:

• NodeRunner.init XMSS verifier setup failure now relies on the existing errdefer self.thread_pool.deinit(), so the thread pool is not deinitialized twice.
• BeamNode.init now destroys the raw BeamChain allocation only on BeamChain.init failure, and installs the chain.deinit(); allocator.destroy(chain); errdefer only after successful initialization. This removes the double-destroy path on later init errors like startChainWorker() / validator setup.
• Existing network_init_cleanup behavior is preserved, so Network.init still gets cleaned up on downstream init failure.

Validation:

• zig build passed locally on the PR head 6e9a764795b55c3c865d175efeead1428965e696 (/tmp/pr877-zig-build.log, EXIT:0).

Minor non-blocking note: the new comment says the two errdefers would double-destroy “if init failed after chain.* was written”; mechanically, the dangerous case is a later error after BeamChain.init succeeds and before BeamNode.init completes. The code itself is correct, so I wouldn’t block on wording.