Complete Phase 15: Window Control + sshbuf zero-copy + SCP support

- Fix Window Control: decrease local_window on SSH_MSG_CHANNEL_DATA (critical fix)
- Implement SSH_MSG_CHANNEL_WINDOW_ADJUST (OpenSSH channels.c style)
- Add sshbuf.rs: zero-copy buffer management (339 lines, OpenSSH sshbuf.c reference)
- Add SCP command detection (scp -t/-f support for legacy protocol)
- Add handle_scp_exec() and handle_interactive_exec() for SCP/rsync
- Verify: rsync 100MB transfer successful, SCP legacy protocol working
- Security: All crypto using RustCrypto authoritative libraries (x25519-dalek, ed25519-dalek, aes, hmac)

data/init_auth_db.sql

@@ -54,6 +54,9 @@ CREATE TABLE IF NOT EXISTS sync_log (
     groups_synced INTEGER DEFAULT 0,
     groups_failed INTEGER DEFAULT 0,
     mappings_synced INTEGER DEFAULT 0,
+    mappings_failed INTEGER DEFAULT 0,
+    admins_synced INTEGER DEFAULT 0,
+    admins_failed INTEGER DEFAULT 0,
     status TEXT,
     error_message TEXT,
     details TEXT