Fix native ChaCha nonce corruption during nested writes

[lzfxxx]

Fix native ChaCha20-Poly1305 nonce corruption during nested writes

Target repository: mscdex/ssh2. Draft for upstream review.
Baseline tested: ssh2 1.17.0, Node 22.20.0, macOS, optional native binding absent.
Updated 2026-10-12: master HEAD a3a39d7
still contains the pattern. GitHub connector reads verified the SHA and searched
open/closed issues and PRs. No equivalent fix was found within the documented
search coverage; see upstream-search.md for limits. Highest tag is v1.17.0;
GitHub Releases is empty, and npm registry publication state was not checked.

Summary

 ChaChaPolyCipherNative.encrypt(packet)
+  allocate a nonce buffer scoped to this invocation
   encrypt packet length
   invoke onWrite(length)
   encrypt payload using the same invocation's nonce

The native implementation uses a module-scoped mutable nonce buffer.
The synchronous onWrite callback can encrypt through another cipher, as
in nested SSH forwarding. That nested call changes the shared sequence
number before the original cipher encrypts its payload. The packet length
is valid, but the peer decrypts an invalid payload.

Use a zero-initialized 16-byte buffer scoped to encrypt(). This isolates
the nonce across different cipher instances in nested callbacks without
changing cipher negotiation or SSH access policy.

Evidence

• Before: unmodified 1.17.0 passes standalone payload validation but fails
  nested service-request payload validation; regression exits 1.
• After: the two-line patch passes standalone and nested payload validation;
  regression exits 0.
• Fixtures use synthetic keys and sequence numbers. No network or credentials.
• The original diagnostic fixture validates payload and packet length.
  The integrated test/test-protocol-chacha-reentrancy.js forces the native
  cipher and decipher, validates authenticated decryption and exact payload,
  and covers standalone/nested writes at sequence numbers 0, 3 and 0xFFFFFFFF.
  It fails on unpatched HEAD and passes all six cases with the patch.
• The complete upstream node test/test.js entry point was rerun on
  2026-10-13 using Node 22.20.0 on macOS with normal approval for localhost
  and test sockets. It exited 1: 12 test files passed, including the native
  six-case regression and upstream crypto; three failed. The OpenSSH 10.3 RSA
  integration test reported client exit 255; server-hostkeys and SFTP tests
  reported localhost AggregateError ETIMEDOUT. These failures have not been
  diagnosed or confirmed against an unpatched baseline in this execution.
  One unsupported OpenSSL cipher case was skipped. This PR does not claim
  the full suite passes.
• Runtime and test subtrees were verified by Git tree hash against HEAD before
  patching. Existing local dependencies were copied; no software was installed.
  Compiled optional binding and Windows/Linux were not tested. Lint was not run
  because ESLint and the upstream config package are unavailable locally.

Merge Danger

Door: two-way. Revert the invocation-local allocation to undo the change.

Blast Radius: cryptography.

One small buffer allocation per packet on this native path. Benchmarking and
upstream review are appropriate. Optional compiled binding and other ciphers
are unchanged. This does not claim to make same-instance recursive encryption
or all module-scoped scratch storage generally reentrant.