Speak protocol 3.0 and fall back to 2.0 only when that is the reflector's maximum.

A reflector that has moved to 3.0 will not accept a station that always announces 2.0, and a 2.x reflector still needs one explicit plaintext session.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/adr/0004-ca-bundle-trust-on-first-use.md

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+# Trust the first CA bundle
+
+Protocol 3.0 checks the reflector with the CA bundle in the certificate directory. When that file is missing, the station stores the bundle after the signature check with the certificate inside the same message. A later digest that differs from the stored file leaves the file in place. Replacing a stored bundle automatically was rejected because the reflector could swap the trust anchor.

docs/adr/0005-protocol-3-with-explicit-2-fallback.md

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+# Announce protocol 3.0, fall back only on an explicit 2.x maximum
+
+Every TCP connection announces protocol 3.0. The station opens one 2.0 session only when the reflector sends a downgrade, or an unsupported-version error, whose maximum is at least 2.0 and below 3.0. The connection after that 2.0 session ends announces 3.0 again. A maximum whose major is below 2 stops the station with the reflector’s message. A reflector `MsgError` stops the station until the operator quits. A bad CA signature, a TLS or hostname failure, a heartbeat timeout, and a dropped connection retry on 3.0, and the window keeps the reason between attempts. The Protocol line shows 2.0 or 3.0 while the session is up, so a plaintext session is visible. Staying on 3.0 after every failure was rejected because a reflector that still expects 2.0 would never accept the station. Falling back on any handshake failure was rejected because a TLS or CA problem would then send plaintext audio.