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>

package/flatpak/org.testium.Testium.yaml

@@ -28,6 +28,20 @@ build-options:
 
 modules:
   - python3-requirements.json
+
+  # Language-server deps for `testium lsp` (pygls + lsprotocol + cattrs + attrs
+  # + typing_extensions). Installed from PyPI at build time — the build already
+  # runs with --share=network (see build-options). The core runtime deps stay
+  # offline-pinned in python3-requirements.json; these are pure-python wheels,
+  # hence --only-binary=:all: (no compilation, deterministic).
+  - name: python3-lsp
+    buildsystem: simple
+    build-options:
+      build-args:
+        - --share=network
+    build-commands:
+      - pip3 install --prefix=${FLATPAK_DEST} --only-binary=:all: "pygls>=1.3"
+
   # 1. Dépendances Python tierces (HORS PySide6)
   # Utilisez flatpak-pip-generator pour vos autres libs (ex: pyserial, requests, etc.)
   # - name: python3-requirements