Restructure: consolidate everything inside testium/ package

Move src/lib/ → src/testium/runtime/ (internal plumbing)
Move src/testium/libs/ → src/testium/api/ (public SDK for test scripts)
Move src/py_func/ → src/testium/py_func/ (Python subprocess)
Move src/lua_func/ → src/testium/lua_func/ (Lua subprocess data)

The package now ships as a single coherent unit instead of four sibling
top-level packages (testium, lib, py_func, lua_func) — pip install
gives a clean site-packages/testium/ with no namespace pollution; .lua
files travel with the wheel via package_data; the wheel installs
cleanly and `testium -b` runs end-to-end including py_func subprocesses
and entry-point exporter plugins.

Naming:
- runtime/ (internal, no API guarantees) clearer than lib/
- api/ (public SDK consumed as `import api.testium as tm`) clearer than libs/

Imports updated en masse: from lib. → from runtime. and from libs. →
from api., plus the importlib.import_module("libs.*") strings in
test_item_console.py and test_item_runtime_plot.py. Test/example
scripts (helper_lib.py, parallel.py, post_execution.py) and the
fake_exporter test suite migrated too.

paths.py: subproc_path() now returns testium_path() — both point at
the testium package directory since the subprocesses live inside.

pyproject.toml: removed exclude=["lua_func", "py_func"] (no longer
needed), added package-data for testium.lua_func/*.lua, removed the
license classifier (PEP 639 conflict with license expression).

Subprocess isolation contract: py_func/ and lua_func/ may only import
runtime/ and their own modules — never interpreter/, main_win/, api/,
or testium/. Enforced by test/validation/items/isolation/ which runs a
py_func that statically scans subprocess source files for forbidden
imports. The contract holds today; the test prevents future drift.

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

CLAUDE.md

@@ -109,9 +109,28 @@ All dialog items (`dialog_image`, `dialog_question`, `dialog_references`, `dialo
 | `src/testium/interpreter/test_items/test_item_parallel.py` | `parallel` and `parallel_branch` items |
 | `src/testium/interpreter/utils/globdict.py` | Global variable dict |
 | `src/testium/interpreter/utils/termlog.py` | Terminal color output |
-| `src/lib/stdout_redirect.py` | `StdioRedirect` singleton (`stdio_redir`) |
-| `src/lib/string_queue.py` | Thread-safe string buffer used for stdout redirection |
-| `src/testium/libs/testium.py` | Public API for test scripts (`tm.*`) |
+| `src/testium/runtime/stdout_redirect.py` | `StdioRedirect` singleton (`stdio_redir`) |
+| `src/testium/runtime/string_queue.py` | Thread-safe string buffer used for stdout redirection |
+| `src/testium/api/testium.py` | Public API for test scripts (`tm.*`) |
+| `src/testium/py_func/` | Python subprocess for `py_func` items (sandboxed: imports only `runtime/` and `py_func/`) |
+| `src/testium/lua_func/` | Lua subprocess scripts for `lua_func` items |
+
+## Package layout
+
+The whole project is a single Python package under `src/testium/`:
+
+```
+src/testium/
+├── __init__.py / __main__.py
+├── runtime/      internal plumbing (jrpc, stdout_redirect, string_queue, tum_except, api)
+├── api/          public SDK exposed to test scripts (`import api.testium as tm`)
+├── interpreter/  test execution engine (NOT visible to py_func/lua_func)
+├── main_win/     GUI (NOT visible to py_func/lua_func)
+├── py_func/      subprocess code for python_func items
+└── lua_func/     subprocess scripts for lua_func items (data files)
+```
+
+`subproc_path()` and `testium_path()` both return the package directory. The py_func subprocess is launched with cwd=that directory and `python3 py_func`. The contract that `py_func/` and `lua_func/` only depend on `runtime/` (no `interpreter`, `main_win`, `api`, `testium`) is enforced by `test/validation/items/isolation/`.
 
 ## GUI icons (main_win)