perf: only do spy and cache work when it will be used

Spy notification ran per recipient what it could run once: both handlers
looked up general.spyMessage and rebuilt the hover component inside the
forEach, and the direct message path also allocated a Set inside the
filter predicate, once per spy per message. It formatted the spy copy of
the message before discovering there were no spies to send it to - and
spies are rare, so that was the normal case.

SpyPermissionManager.notifySpies now owns the whole shape. It takes the
body as a lambda so nothing is built for an empty audience, attaches the
hover once, and reads the roster directly rather than through
getDirectMessageSpyPlayers()'s defensive copy. It also puts "who must not
see this" in one place; the two handlers had drifted to expressing it by
name in one and by UUID in the other.

ConversionCache tracked no dirtiness, so the periodic task re-serialized
and rewrote the entire cache file on its fixed interval whether or not
anyone had chatted, while every single put scheduled another full rewrite
five seconds out. It now records that it changed and the periodic task is
the only writer, returning immediately when there is nothing to write. A
failed write marks the cache dirty again so the next tick retries.

Co-Authored-By: Claude <noreply@anthropic.com>

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