lsp: declarative action registry + cross-channel language server

Make `testium lsp` (and the testium_assist editor extension that spawns it)
work from every distribution channel: source, wheel, PyInstaller, Flatpak,
AppImage.

Two enablers:

1. Declarative ACTIONS registry. The TestItemActions parents (console, plot,
   json_rpc) now declare their nested actions as a class attribute
   `ACTIONS = {yaml_key: class}`, mirroring PARAMS. The base __init__ seeds
   action_classes from type(self).ACTIONS; register_actions() is kept only as
   an imperative escape hatch. lsp/schema.py reads ACTIONS directly, dropping
   the inspect.getsource/AST walk that returned no actions in a frozen
   PyInstaller build (no .py source on disk).

2. pygls bundled per channel. Kept as the pyproject [lsp] extra (lean
   `pip install testium`), layered into each full-app channel:
   - build_env.sh installs pygls into test/tmp/.venv (source run + PyInstaller
     build env)
   - AppImage installs the wheel as `…whl[lsp]`
   - Flatpak adds a python3-lsp network-pip module (matches the manifest's
     global --share=network)
   - PyInstaller .spec collect_submodules(pygls/lsprotocol) + hiddenimports for
     the lazily-imported lsp/lsp.server/lsp.schema

test/validation/lsp_smoke.py (run by run.sh before the suite) enforces both
per channel: `<channel> schema` must keep console/plot/json_rpc actions and
`<channel> lsp` must answer an initialize request without reporting pygls
missing. Verified for source mode; the other channels need a rebuild to verify.

DESIGN.md updated (declarative section + new "Language server across channels"
subsection + Recent fixes).

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

scripts/build_env.sh

@@ -33,6 +33,11 @@ if [ ! -d "$PY_VENV_DIR" ]; then
     python3 -m venv "$PY_VENV_DIR"
     source "$PY_VENV_DIR/bin/activate"
     pip install --extra-index-url https://pypi.python.org/pypi -r $REQ_PATH
+    # Language-server deps (the pyproject [lsp] extra). Installed here so the
+    # source run AND the PyInstaller build — both of which use this venv — can
+    # start / collect the `testium lsp` server. pip-installed wheel users get
+    # them via `pip install testium[lsp]` instead.
+    pip install --extra-index-url https://pypi.python.org/pypi "pygls>=1.3"
     # Validation suite plugin used to verify the report-exporter
     # entry-points discovery end-to-end.
     FAKE_EXPORTER_DIR="$(dirname "$REQ_PATH")/../test/validation/fake_exporter"