Add /w whispers delivered to one player at any distance

[treestar84]

Add /w whispers delivered to one player at any distance

Implements the TODO item: "귓속말(DM) 기능" (doc/TODO.md)

Demo

Three clients side by side. Miru's normal chat reaches everyone nearby; her /w Rica whisper reaches only Rica — Nami, standing right next to them, sees nothing. Rica replies with /w and the exchange stays private.

Design

Whispers ride the existing chat path end to end:

• Server-side parsing: /w <name> <message> (alias /whisper) is parsed in send_chat_message next to /who and /escape, with the syntax defined once in parse_whisper_command following the parse_notice_command pattern. Because it's plain chat text, the agent-client gets whispers for free through its existing say action — no new client message, same protocol for humans and agents.
• ServerMessage::WhisperMessage { from, to, message } carries names instead of ids: whisper ignores distance, so either end may be outside the other's AOI where an id resolves to nobody client-side. It is delivered only to the target plus an echo to the sender.
• An exact spelling always wins. Names are UNIQUE only case-sensitively ("Rica" and "rica" can coexist), so case-insensitive matching (/w rica) is a convenience that applies only while it is unambiguous — with several case variants online, the sender is asked to spell the name exactly rather than risk a private message reaching a near-miss. Names never contain spaces, so /w <name> <rest> parses unambiguously.
• Errors go only to the sender — unknown name, missing message, self-whisper each get a private reply; a malformed whisper never leaks into local chat.
• Like regular chat, whisper content stays out of logs (F-012) — and so does the recipient: who talks to whom is metadata worth the same protection. Only from/len are logged.
• Adding a ServerMessage variant changes the wire shape, so PROTOCOL_VERSION bumps to 3 (per the bump-on-shape-change rule in shared/src/lib.rs).

Client: whispers render in purple as From Miru: / To Rica:, with no overhead bubble — a whisper is private. /w and /whisper join tab completion.

Agent-client: an incoming whisper classifies as Urgent (the echo of your own is Noise), reaches the LLM as a [Whisper] name: message event, and the system prompt explains how to answer one. This is groundwork for the "전언 시스템" idea — players can already whisper the resident NPCs.

Testing

• New server tests: parse cases (/w, /whisper, /who non-collision, missing parts), delivery to a target 500m away with echo + no bystander + no broadcast leak (mixed-case name, canonical to), coexisting case-variant names (exact spelling wins, ambiguous spelling is refused and delivered to no one), and each error path replying only to the sender.
• Full workspace suite passes; cargo fmt / cargo clippy (0 warnings) / npm run check / npm run lint / npm run test (279 client tests) all clean.
• Verified end-to-end with three local clients (the GIF above), including /w rica lowercase, unknown-name and missing-message replies, /who regression, and tab completion cycling /w → /whisper.