fix(cf): register crashes on first name collision under set -e

[marksverdhei]

Bug

cf register's name-retry loop used ((attempts++)):

attempts=0
while [ $attempts -lt 100 ]; do
    ...
    grep -q "^${MYNAME}:" "$CHATFILE" 2>/dev/null || break
    ((attempts++))           # <-- on the 1st iteration this evaluates to 0
done

((expr)) returns exit status 1 when the arithmetic result is 0, and post-increment attempts++ evaluates to the old value (0) on the first iteration. With the script's set -e, that aborts cf register the moment a generated name collides with an existing participant — instead of retrying for a fresh name.

$ bash -c 'set -e; attempts=0; ((attempts++)); echo survived'   # never prints; exit 1

Fix

• attempts=$((attempts + 1)) — an assignment always returns success, so the loop retries correctly.
• Added a guard: if all 100 attempts collide, error out instead of silently registering a duplicate name (the old loop fell through using the last colliding name).

Test

test_register_survives_name_collisions fills a chunk of the name space so collisions are frequent, then registers 30×; each must succeed with a non-empty name. It fails against the old ((attempts++)) (39/40) and passes with the fix (40/40).

Context

Salvaged from #1, which I'm closing as superseded: it predates the global-room rework (#3 closed issue #2 with a different $HOME/.cf_session approach) and its diff would revert main's CI workflow and presentation-helper tests. This is the one unambiguous, still-applicable fix from it. (Other candidates in #1 — send newline sanitization, tail -f orphan cleanup in await, a LASTLINE read-cursor — are noted on #1 as possible follow-ups.)

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