fix: target standard CPython 3.14 (mitmproxy incompatible with 3.14t)

mitmproxy's aioquic/mitmproxy-rs deps have no free-threaded wheels and
source builds fail. Workload is I/O-bound so free-threading gave no
benefit. Switch .python-version to 3.14, drop /tmp stub overrides,
reinstall mitmproxy with real wheels (imports cleanly). Update README
and design spec to record the decision.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.python-version

@@ -1 +1 @@
-3.14+freethreaded
+3.14