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/pyinstaller/testium.spec

@@ -1,5 +1,21 @@
 # -*- mode: python ; coding: utf-8 -*-
 import os
+from PyInstaller.utils.hooks import collect_submodules
+
+# Language-server dependencies for `testium lsp`. pygls/lsprotocol register
+# converters and features dynamically, so we collect their submodules wholesale
+# and force-import their pure-python deps (cattrs/attrs/typing_extensions).
+# The testium lsp modules are imported lazily by the CLI dispatch
+# (`from lsp.server import serve`), which PyInstaller's static analysis misses —
+# hence the explicit names. No source files need bundling: the schema export is
+# now fully declarative (PARAMS + ACTIONS class attributes), so it no longer
+# reads .py source via inspect.getsource (which fails in a frozen build).
+_LSP_HIDDEN = (
+    collect_submodules("pygls")
+    + collect_submodules("lsprotocol")
+    + ["cattrs", "attr", "attrs", "typing_extensions",
+       "lsp", "lsp.server", "lsp.schema"]
+)
 
 # junit_xml is imported by post_exec scripts running under the *host* Python,
 # not the frozen interpreter — so bundling it via hiddenimports alone is not
@@ -54,7 +70,7 @@ a = Analysis(
                    "colorama",
                    "matplotlib",
                    "junit_xml",
-                   "lxml"],
+                   "lxml"] + _LSP_HIDDEN,
     hookspath=[],
     hooksconfig={},
     runtime_hooks=[],