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>

pyproject.toml

@@ -34,12 +34,6 @@ typeCheckingMode = "strict"
 venvPath = "."
 venv = ".venv"
 
-[tool.uv]
-override-dependencies = [
-    "aioquic @ file:///tmp/aioquic-stub",
-    "mitmproxy-rs @ file:///tmp/mitmproxy-rs-stub",
-]
-
 [dependency-groups]
 dev = [
     "pytest>=9.0.3",