Un-isolate bigcrawl on macOS and bump it to -c16

PR #538's larger listen backlog fixed the real cause of the macOS bigcrawl
file-count flake: Python's default 5-slot accept queue overflowed under parallel
CPU contention, and macOS drops SYNs on overflow. With that gone, macOS no
longer needs the second-pass "run bigcrawl alone" workaround, so it runs the
full suite in parallel like Linux.

The crawl can also push more connections now: -c8 to -c16 shaves ~14% off its
wall time (33.4s to 28.6s locally; -%c100 caps the rest) and stresses the engine
harder. 128 backlog slots leave ample room for 16 connections.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

.github/workflows/ci.yml

@@ -135,14 +135,6 @@ jobs:
       - name: Build
         run: make -j"$(sysctl -n hw.ncpu)"
 
-      - name: Add loopback aliases (macOS lacks 127.0.0.2/.3)
-        # 19_local-connect-fallback needs the dead 127.0.0.2/.3 to refuse
-        # instantly like Linux; alias them onto lo0 so they don't stall to timeout.
-        run: |
-          set -euo pipefail
-          sudo ifconfig lo0 alias 127.0.0.2 up
-          sudo ifconfig lo0 alias 127.0.0.3 up
-
       - name: Test
         run: |
           jobs=$(( $(sysctl -n hw.ncpu) * 2 )); [ "$jobs" -le 16 ] || jobs=16