feat: add SSH config runtime foundation (#295)

[inureyes]

Summary

• Add the ssh_config runtime foundation for the first-wave compatibility tracker with a canonical accepted-directive registry, structured Include provenance, OpenSSH first-obtained resolution, and generic command-line overlays.
• Apply validated transport algorithms, bounded carrier-only retries, verified TCP keepalive, and typed packet-integrity diagnostics without retrying or relabeling host-key, authentication, channel, or command failures.
• Keep all remaining first-wave behavior explicitly delegated to feat(ssh): enforce ssh_config authentication policy and user certificates #296 through feat(ssh): enforce RekeyLimit through russh transport limits #301 instead of silently claiming runtime support.

Changes

• Classify every accepted keyword and alias as a concrete runtime consumer, delegated issue, or recognized unimplemented option, with invariants that reject duplicate or unsupported runtime claims.
• Preserve actual nested Include path and line metadata in an internal structure, deduplicate canonical diagnostics once per parse, and route them through stderr or -E without trusting user-authored # Source: comments.
• Resolve pre-Host globals, nested Include source order and caller context, Host/Match blocks, repeated -o Key=Value values, scalar first-obtained fields, additive fields, and SetEnv key-level first writes.
• Implement OpenSSH replacement, append, remove, prepend, wildcard, order, deduplication, supported-name validation, fail-closed empty policies, and russh-required KEX extension preservation for Ciphers, MACs, KexAlgorithms, HostKeyAlgorithms, and PubkeyAcceptedAlgorithms.
• Retry only DNS and TCP carrier creation for ConnectionAttempts, preserve the last typed source on exhaustion, apply TCPKeepAlive as a verified OS socket option, and preserve typed russh packet authentication, AEAD, packet-size, and padding failures in connection-local take-once state.
• Share integrity state only within clones of one direct, ProxyCommand, jump, interactive, forwarding, command, or file-transfer Client; each new connection, reconnect, retry, jump hop, and destination gets a fresh state.

Compatibility notes

• This PR includes only the -c and -m portion of feat(cli): implement -c, -m, -s, -n and -W #285 ahead of its normal phase because the feat(ssh_config): add directive registry and transport runtime foundation #295 named try-ciphers and integrity regressions invoke those OpenSSH flags directly; dedicated flags precede generic -o Ciphers and -o MACs, and feat(cli): implement -c, -m, -s, -n and -W #285 must not implement them again.
• The feat(ssh_config): add directive registry and transport runtime foundation #295 registry contract intentionally replaces the incidental fix(ssh_config): execute ProxyCommand instead of parsing and discarding it #280-era assertion that SecurityKeyProvider was silent: it remains accepted but unimplemented and now emits one canonical provenance-aware warning, while the completed ProxyCommand behavior from fix(ssh_config): execute ProxyCommand instead of parsing and discarding it #280 remains unchanged.
• Remote disconnect descriptions are not trusted as integrity evidence; only typed russh packet-decoding causes select the integrity diagnostics.

Validation

• cargo test --lib ssh::ssh_config (230 passed)
• cargo test --lib ssh::tokio_client::connection_tests (26 passed)
• Focused algorithm, typed-error, connection-state, CLI, jump-chain, and provenance diagnostic tests (46 passed)
• make openssh-regress-selftest (13 passed)
• cargo fmt --all -- --check
• cargo check --lib --bins --tests
• cargo clippy --lib --bins --tests -- -D warnings
• OpenSSH regressions integrity, try-ciphers, kextype, and keytype (4/4 passed)

Closes #295

Part of #281 and #275