modes.rst: new 'Language server (editor support)' section — testium lsp / testium schema, the testium_assist client, the [lsp] extra. tum_syntax.rst: a note on parameter validation (unknown-key warning, missing-required error). Builds clean (sphinx html, no RST errors). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>