Four more modules into Nim: members, glyphs, emoji, store

Not deleted from `common/` — CLAUDE.md is explicit that the ClojureDart
original stays as the web's implementation until there is a wasm build, and
deleting one would take the web target with it. This is the port, not a
removal.

`members` brought a model change with it. `Room.users` was a list of names and
is now nick → mode prefix, because the panel sorts ops first and a MODE has to
find one person among many. With it: 353 accumulating into a pending list and
366 promoting it — replacing the list on each 353 empties the panel and refills
it a name at a time — and MODE, which is the fiddly one. A mode that takes an
argument without naming a member still eats one, so `+ko secret alice` ops
alice rather than putting the op on whoever `secret` matched. There is a test
for exactly that.

`emoji` is generated, not typed. `tools/emoji2nim.py` reads the Clojure and
emits the Nim — 1,884 entries, matching row for row — because the catalogue is
Unicode's own list and a second hand-maintained copy is how two languages
start disagreeing about what can be drawn. `just test emoji` regenerates it.

`glyphs` splits a line into words and pictures on top of that. Longest match
first, because 👨 is the head of 👨‍👩‍👧 and taking the man would leave his
family as three more glyphs and two tofu joiners; and a skin tone is stripped
for the lookup and kept for the drawing, since the catalogue leaves toned
variants out and the pack has them.

`store` is JSON rather than EDN. The Clojure writes through the `frq.io` seam
because ClojureDart had no portable filesystem; Nim has one, so the seam is
gone — and nothing but this reads these files, so a format with a parser in the
standard library is one less thing to get wrong. Every read treats a file that
will not parse as absent: a stale credential is not worth an error at startup,
and a room with a broken marker would count its whole history unread. The
session file is created and then restricted, because there is no atomic
create-with-mode here and the alternative is a token written world-readable and
never narrowed.

Three of my own tests were wrong before the code was: `completeNick("bo", …)`
at position 0 really is the start of a line, so `bob: ` is right and `bob ` was
my expectation rather than the rule.

258 Nim tests, 20 Dart, 21 layout, no warnings, and the window still reaches
#test with 114 lines and no layout errors.

Remaining Clojure logic with no Nim yet: msgsig and crypto (ed25519, which
needs a dependency or an implementation — the core has no dependencies outside
Nim's stdlib today), profile, and replies.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>