fix(windows): real toolchain probing, HOME/TEMP env, and gate portability

code_run.py: shutil.which() finding a compiler executable on PATH doesn't
mean it's a usable toolchain on Windows -- rustc's MSVC target also needs
Microsoft's linker, and an MSYS2 gcc/clang driver can remain resolvable after
one of its runtime DLLs has broken. Both cases silently turned every C/C++/
Rust snippet into a compile error while the capability check said "ready".
_compiled_tool() now actually compiles+links a trivial known-good program
per candidate (Windows only; POSIX keeps the cheap which(1) check since
release hosts install compiler packages atomically) and caches the result.

Also fixes the run/compile child environment: HOME/TMPDIR don't control
Windows' real temp/profile resolution (expanduser() reaches the actual user
profile, GetTempPath() falls back to the Windows directory), letting a
snippet escape the scratch directory or fail outright. _child_env() now also
sets TEMP/TMP/USERPROFILE on Windows.

tests/conftest.py (new): isolates curry_store's SQLite singleton into a
per-run temp directory via NEXUS_CURRY_DB before any test module imports
synapse, and cleans it up at session end -- the release gate no longer
writes test constants into the checkout's live data/curry.db. .gitignore
picks up /data/curry.db for whatever still lands there locally.

bin/check.sh: falls back to Promethean/Scripts/python.exe when
Promethean/bin/python doesn't exist, so the gate actually runs on a Windows
venv instead of immediately exiting "no Promethean venv".

Verified independently: 254 passed, 0 failed, 8 skipped (tests + management)
-- the 12 C/C++/Rust toolchain failures present all session are gone. Full
bin/check.sh run end-to-end on this Windows checkout: pytest, eslint,
frontend node:test (57/57), PowerShell/shell parse, and the wheel/sdist
packaging + twine + content checks all report OK.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/conftest.py

@@ -0,0 +1,23 @@
+"""Test-process isolation for persistent runtime state."""
+from __future__ import annotations
+
+import os
+import shutil
+import sys
+import tempfile
+from pathlib import Path
+
+
+# curry_store constructs its SQLite singleton during test collection. Point it
+# at a per-run directory before any test module imports synapse, so the release
+# gate is repeatable and never writes test constants into the checkout's live
+# data/curry.db.
+_TEST_STATE = Path(tempfile.mkdtemp(prefix="nexus-pytest-"))
+os.environ["NEXUS_CURRY_DB"] = str(_TEST_STATE / "curry.db")
+
+
+def pytest_sessionfinish(session, exitstatus):
+    module = sys.modules.get("synapse.curry_store")
+    if module is not None:
+        module.curry_db.close()
+    shutil.rmtree(_TEST_STATE, ignore_errors=True)