fix(cli/tests/dm): bind relay to 127.0.0.2 + dm-06 → --since filter

Two fixes needed to make the DM harness actually pass end-to-end:

1. Bind the loopback relay to 127.0.0.2 instead of 127.0.0.1.

   Quartz's `RelayTag.parse` (called from `ChatMessageRelayListEvent.
   relays()`) rejects any URL for which `isLocalHost()` returns true —
   the substring check matches `"127.0.0.1"` among others. That means a
   kind:10050 event whose `relay` tag reads `ws://127.0.0.1:8090/`
   silently parses to an empty relay list, and `RecipientRelayFetcher`
   reports `dmInbox == []`. Under strict mode `dm send` then correctly
   refuses with `no_dm_relays` — hiding an otherwise-valid test setup.

   `127.0.0.2` is the cleanest fix: still pure loopback (no network
   traffic, no config needed), not matched by `isLocalHost()`, and
   Linux binds any IP in 127.0.0.0/8 without setup. The Marmot harness
   is unaffected because its tests never depend on parsing their own
   kind:10051/10002 relay tags back out — they hit the bootstrap
   fallback path.

   marmot/setup.sh's `start_local_relay` now reads `${RELAY_HOST:-
   127.0.0.1}` so both harnesses can share it; the DM harness sets
   `RELAY_HOST=127.0.0.2` by default and exposes `--host` as an escape
   hatch.

2. Replace dm-06 cursor-advance check with `--since` filter check.

   The original assertion ("second no-flag `dm list` returns 0") was
   wrong for the actual design: `filterGiftWrapsToPubkey` always
   subtracts a 2-day lookback from `since` (required to catch NIP-59's
   randomised `created_at`), so a repeat no-flag query legitimately
   re-pulls everything in the last two days. The cursor advances the
   scan window for efficiency, not for idempotency. dm-06 now verifies
   `--since` actually filters: query with `--since <newest + 2 days +
   1 hour>` — after the filter's lookback subtraction, the effective
   floor lands past every message, and the list comes back empty.

Also: added `protoc` to the DM preflight checks so the missing-dep
error comes before 4 failed cargo retries.

Result on this machine: 6/6 pass, two clean runs in a row.

cli/tests/marmot/setup.sh

@@ -170,7 +170,7 @@ description = "Loopback relay for marmot-interop-headless.sh — do not use for
 data_directory = "$RELAY_DATA"
 
 [network]
-address = "127.0.0.1"
+address = "${RELAY_HOST:-127.0.0.1}"
 port = $RELAY_PORT
 
 [options]
@@ -198,7 +198,7 @@ EOF
 
   local deadline=$(( $(date +%s) + 20 ))
   while [[ $(date +%s) -lt $deadline ]]; do
-    if curl -sSf -m 1 "http://127.0.0.1:$RELAY_PORT/" >/dev/null 2>&1; then
+    if curl -sSf -m 1 "http://${RELAY_HOST:-127.0.0.1}:$RELAY_PORT/" >/dev/null 2>&1; then
       info "relay up"
       return 0
     fi