fix(openssh): close residual regress compatibility gaps

[inureyes]

Part of #275. Follow-up to #313.

Context

The final OpenSSH V_10_3_P1 compatibility runs for #313 reached the epic gate on both CI platforms:

• macOS: 60 passed out of 66 eligible tests, 6 failed, 7 environmental, 6 skipped
• Ubuntu: 61 passed out of 69 eligible tests, 8 failed, 4 environmental, 6 skipped

This issue keeps every remaining candidate-only failure explicitly tracked after the epic closes. It does not reclassify environmental failures or reduce the committed score floor.

Remaining candidate failures

Test	macOS	Ubuntu	First observed gap
agent-restrict	fail	fail	candidate key generation path
dynamic-forward	fail	fail	dynamic forwarding startup and SOCKS coverage
hostkey-agent	fail	fail	Ed25519 host-key agent behavior
multiplex	fail	fail	unsupported -O conninfo and remaining control operations
percent	fail	fail	percent expansion in match/exec handling
proxyjump	fail	fail	basic parsing and multi-hop behavior
rekey	environmental	fail	Ubuntu candidate timeout
scp3	environmental	fail	Ubuntu remote-to-remote copy behavior

The uploaded result tables classify channel-timeout, connection-timeout, forwarding, and sftp-badcmds as environmental on both platforms. macOS additionally classifies forward-control, rekey, and scp3 as environmental because the reference client fails in the same environment. The six upstream skips remain unchanged.

Scope

• reproduce each candidate-only failure against the pinned harness
• fix compatibility gaps without weakening fixture validation or score floors
• split independent implementation areas into linked issues when a focused fix is safer
• retain reference-client evidence for platform-specific environmental classifications

Acceptance criteria

• Every listed candidate-only failure either passes on both supported CI platforms or has a narrower linked issue with reproduction evidence.
• The committed eligible-result floors are not lowered to hide a regression.
• macOS and Ubuntu result artifacts identify all remaining fail, environmental, and skip verdicts.