MXP negotiation parity: C, Kotlin, and Swift client

[powerje]

The Swift server gained MXP negotiation (option 91) earlier; this adds it to the rest.

Implementation	Change
C (cmth)	COMM_FLAG_MXP (BV09); telnet_table[91] → ANNOUNCE_WILL; process_do_mxp (set flag, emit ESC[7z locked default) + process_dont_mxp, wired into negotiation_table
Kotlin server (mth-core/server)	CommFlags.MXP (1 shl 8); table[91] → AnnounceFlags.WILL; processDoMxp/processDontMxp + dispatch; mxpEnabled / reassertMXP()
Kotlin client (mth-core/client)	responds to WILL MXP with DO MXP; mxpEnabled
Swift client (SwiftMTHClient)	responds to WILL MXP with DO MXP; mxpEnabled

Server side announces WILL MXP, handles the client's DO/DONT, and on enable emits ESC[7z (Lock Locked) so normal output is never parsed as markup — matching the Swift server. Client side accepts the server's WILL with DO and tracks the flag (tag rendering remains the client app's job).

Tests

• Swift client: mxpWillIsAcceptedWithDo (feeding WILL MXP sets mxpEnabled + writes DO MXP).
• Kotlin server: announce WILL MXP, DO→enabled+ESC[7z, DONT→disabled.
• Kotlin client: WILL MXP→DO MXP+enabled.
• Swift swift test (155) and Kotlin :mth-core:test both green; C compiles via swift build.