validation: dedicated venv + fix python_bin override timing

eval_proc was started before -d/GUI defines reached gd, so
``-d python_bin=...`` and the GUI ``python_bin`` preference were
silently ignored by the very subprocess that runs ``<| ... |>`` evals
(and only took effect for later items once the discovery cache had
already been seeded with the system interpreter). apply_overrides() is
now applied before eval_process_init(), and bins._resolve()'s cache is
keyed by (name, override) so a later param.yaml change re-resolves on
the next lookup.

The validation suite now ships a wrapper (run.sh / run.bat) that
creates a dedicated venv in the system temp dir and pins it via
``-d python_bin=...``. A new ``venv`` item asserts the override took
effect for both eval_proc and py_func paths, with a
``sys.prefix != sys.base_prefix`` marker to catch the case where the
override happens to be a system interpreter (path-equality alone would
miss it, the venv's ``bin/python3`` being a symlink to the host).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

test/validation/run.bat

@@ -0,0 +1,61 @@
+@echo off
+SETLOCAL EnableExtensions
+
+REM Runs the testium validation suite with a dedicated Python venv used
+REM by every py_func / cycle / inline-eval subprocess. testium itself
+REM keeps running in the project's own environment; the validation venv
+REM only isolates *test execution*.
+REM
+REM   test\validation\run.bat [clean] [extra testium args]
+REM
+REM Requires the project venv to already exist (run the project's
+REM run.bat once first, or any other testium install method).
+
+SET "SCRIPT_DIR=%~dp0"
+SET "PROJECT_DIR=%SCRIPT_DIR%..\.."
+REM Venv in the user temp dir (Windows equivalent of /tmp).
+SET "VENV_DIR=%TEMP%\testium-validation-venv"
+SET "PROJECT_VENV=%PROJECT_DIR%\test\tmp\testium_venv"
+
+IF /I "%~1"=="clean" (
+    rmdir /s /q "%VENV_DIR%"
+    SHIFT
+)
+
+REM Locate a host Python.
+SET "PYTHON_EXE=python"
+py --version >nul 2>&1
+IF %ERRORLEVEL% EQU 0 (
+    SET "PYTHON_EXE=py"
+    goto :PYTHON_FOUND
+)
+python --version >nul 2>&1
+IF %ERRORLEVEL% EQU 0 (
+    SET "PYTHON_EXE=python"
+    goto :PYTHON_FOUND
+)
+echo ERROR : Python could not be found on this system.
+exit /b 1
+
+:PYTHON_FOUND
+
+IF NOT EXIST "%VENV_DIR%" (
+    echo Creating validation venv at %VENV_DIR%
+    %PYTHON_EXE% -m venv --system-site-packages "%VENV_DIR%"
+    IF %ERRORLEVEL% NEQ 0 (
+        echo ERROR while creating the validation venv.
+        exit /b 1
+    )
+    call "%VENV_DIR%\Scripts\pip" install --quiet --upgrade pip
+    call "%VENV_DIR%\Scripts\pip" install --quiet junit-xml
+)
+
+SET "VENV_PYTHON=%VENV_DIR%\Scripts\python.exe"
+
+IF NOT EXIST "%PROJECT_VENV%" (
+    echo ERROR : project venv not found at %PROJECT_VENV%. Run the project run.bat once first.
+    exit /b 1
+)
+
+call "%PROJECT_VENV%\Scripts\activate"
+python "%PROJECT_DIR%\src\testium" -b -d "python_bin=%VENV_PYTHON%" -- "%SCRIPT_DIR%main.tum" %*