fix: free retrieval hint with libc free, not deallocShared (SIGSEGV)

[Ivansete-status]

Crash

A SIGSEGV surfaced in CI (status-go PR-7378):

wrapOutgoingMessage -> getRecentHistoryEntries -> onRetrievalHint (libsds.nim)
  -> dealloc -> rawDealloc -> deallocBigChunk -> avltree.del
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Root cause

The retrieval-hint provider (the Go binding's C.CBytes) allocates the returned hint with malloc and hands ownership to us. The hand-written hint closure freed it with deallocShared — i.e. through Nim's shared-heap allocator, not libc free. Freeing a malloc'd pointer through Nim's allocator corrupts that allocator's metadata, so a later allocation crashes in deallocBigChunk/avltree.del. Once wrapOutgoingMessage starts invoking the provider per causal-history entry, this triggers reliably.

The hint provider is hand-written (a raw C function pointer has no JSON form for nim-ffi to marshal), so this free is nim-sds's responsibility.

Fix

Free the malloc'd buffer with libc free.

Verification

Reproduced end-to-end via the C ABI on the release/v0.3 line (a malloc-based provider + 500 SdsWrapOutgoingMessage calls, ~4900 provider invocations): the buggy build segfaults (exit 139), the fixed build completes cleanly. Same one-line allocator fix applies here. nim check passes.

Companion fixes: nim-sds v0.3.3 (release/v0.3) and PR #86 (release/v0.4) carry the identical fix.

🤖 Generated with Claude Code