fix(telnet): stop Mudlet masking all input for the whole session

[MorquinDevlar]

Problem

When connecting with Mudlet (raw telnet, not the web client), the input line is masked (hidden) for the entire session - the username prompt, every command, everything - not just password prompts.

Root cause

On connect, handleTelnetConnection() unconditionally sent IAC WILL ECHO to every telnet client and never withdrew it. GoMud runs in character-at-a-time mode and does server-side echo (echoing each typed character itself, emitting mask characters for passwords). That is correct for raw character-mode telnet clients that rely on the server to echo.

Mudlet, however, echoes locally and interprets the telnet ECHO option purely as a password-masking hint:

• IAC WILL ECHO -> hide/mask the input line
• IAC WONT ECHO -> show the input line

Because the server asserted WILL ECHO once at connect and never sent WONT ECHO (its masking is done at the application layer), Mudlet stayed in "masked" mode permanently.

Fix

Detect the client type at connect and branch the echo behavior. Raw-telnet and web-client behavior are unchanged; the new behavior applies only to local-echo (Mudlet) clients.

State	Mudlet baseline	During password prompt	After password prompt
Server sends	IAC WONT ECHO	IAC WILL ECHO	IAC WONT ECHO
Mudlet input	visible (local)	masked	visible again

Detection (the timing problem)

GoMud does not know a client is Mudlet at the moment it must choose the connect-time echo baseline (GMCP detection arrives too late). This adds a short, synchronous MNES NEW-ENVIRON probe before the first prompt: the server requests CLIENT_NAME/CLIENT_VERSION, and Mudlet replies CLIENT_NAME=Mudlet. The probe is bounded (200ms per read, 2s overall); a client that ignores NEW-ENVIRON falls back to non-Mudlet and still logs in. AI connections skip the probe and keep the historical baseline.

Changes

• internal/connections/clientsettings.go - add IsMudlet / DetectionComplete.
• internal/connections/connectiondetails.go - SetReadDeadline for bounded probe reads (telnet path only).
• internal/term/{telnet,term}.go - NEW-ENVIRON sub-negotiation codes, request/response matchers, and a TelnetRequestMNESVars helper.
• internal/inputhandlers/term_iac.go - negotiate NEW-ENVIRON and record IsMudlet from CLIENT_NAME (+ unit test for the response parser).
• main.go - detectClientType probe and the echo-baseline branch; the login handler is now added to the chain after detection.
• internal/inputhandlers/login_prompt_handler.go - skip server echo/mask and the Enter newline for local-echo clients; toggle WILL/WONT ECHO masking around Mudlet password steps.

Test plan

• Mudlet: username prompt shows typed text; password prompt masked; after login all gameplay input visible. Reconnect a few times (timing).
• New-character creation in Mudlet: new-password and verify-password steps mask; everything else visible.
• Raw telnet (telnet/tintin++): unchanged - server echoes input, passwords masked with mask chars.
• Web client: unchanged - TEXTMASK toggles the password field; no double echo.
• Detection timeout: a client that ignores NEW-ENVIRON falls back to non-Mudlet within ~2s and still logs in.

Automated: make validate and go test ./... pass; added TestNewEnvironIsMudlet / TestNewEnvironResponseMatcher.

[Volte6]

One small change request.

This is a lot of change for mudlet, with special exceptions. That's annoying. Oh well.

[Volte6]

These IACByte's are already defined - should be aliases:

	TELNET_NEWENV_IS      = TELNET_OPT_TXBIN
	TELNET_NEWENV_SEND    = TELNET_OPT_ECHO
	TELNET_NEWENV_INFO    = TELNET_OPT_RECONN
	TELNET_NEWENV_VAR     = TELNET_OPT_TXBIN
	TELNET_NEWENV_VALUE   = TELNET_NEWENV_SEND
	TELNET_NEWENV_ESC     = TELNET_NEWENV_INFO
	TELNET_NEWENV_USERVAR = TELNET_OPT_SUP_GO_AHD

[MorquinDevlar]

Superseded by #633, re-opened from a fork (MorquinDevlar/GoMud). Closing this branch-based PR; the diff is identical.