plug memory leaks on error paths (adversarial review)

[ch4r10t33r]

Summary

Adversarial review of the codebase surfaced a cluster of rare-error-path memory leaks and one undefined-behavior case. All of them share the same shape: an allocation succeeds but a follow-up step (usually ArrayList.append, a validation check, or a secondary allocation) fails and the intermediate allocation escapes cleanup.

Fixes

• broadcast/channel_rs - publish / attachRelaySession allocated member_ids and stats arrays, then populated them in a loop. The errdefer iterated the full array, so a mid-loop allocator.dupe failure would free uninitialized slots (UB). Track a populated counter and only free member_ids[0..populated].
• wire/broadcast - decodeBcast / decodeSess decoded a nested inner buffer and then rejected duplicate oneof fields without freeing it. Free inner before returning error.MalformedProtobuf.
• wire/rs - decodePreamble leaked the per-iteration hash buffer if hashes.append failed, and leaked the owned hashes slice if the final default-hash dupe failed. Add an errdefer for the per-iteration buffer and restructure the return so hashes_owned is covered before the default-hash allocation.
• discv5/protocol - Six encoders (encodePing, encodePong, encodeFindNode, encodeNodes, encodeTalkReq, encodeTalkRes) used the leaky try items.append(a, try enc(a, ..)) pattern. Introduce an appendOwned helper that frees the buffer on append failure and route every append through it. Also add errdefer cleanup to decodeFindNode / decodeNodes so partially populated temporary lists are freed on error.
• discovery/enr - Same nested-try pattern in buildContent and sign. Add an appendOwnedItem helper and route all appends through it.
• transport/zquic_quic_shim - streamMake / streamMakeUni leaked the QuicStream allocation if conn.streams_owned.append failed. Add errdefer alloc.destroy(qs) and errdefer qs.deinit() between create and append.
• sim/gossipsub_rpc_pb - decodeIHaveOwned, decodeIWantOwned, decodePruneOwned and decodeRpcOwned leaked the freshly decoded child (duped bytes, or an owned sub-struct with its own nested allocations) when the outer list append failed. Register an errdefer on the intermediate value before appending.

No public API changes, no dependency changes, no behavior change on the happy path.

Test plan

• zig fmt --check .
• zig build test (150 passed)
• zig build test-broadcast (54 passed)
• zig build test-sim-rs (22 passed)
• zig build test-sim-gossipsub (23 passed)
• zig build test-quic (44 passed)