Add interactive tree editor island (Phase 4) + MCP server fixes

Phase 4: defisland ~sx-tools/tree-editor — interactive tree viewer
embedded in the SX Tools page. Features:
- Textarea with :bind for SX source input
- Parse button to re-parse on demand
- Tree view: annotated tree with path labels, clickable nodes
- Context view: enclosing chain from root to selected node
- Validate view: structural integrity checks (catches missing body etc.)

MCP server fixes: added ident-start?, ident-char?, make-keyword,
escape-string, sx-expr-source — needed by parser.sx when loaded
into the MCP evaluator.

Also: .mcp.json for Claude Code MCP server config, CLAUDE.md protocol
for structural .sx file editing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/sx-tools.sx

@@ -195,6 +195,11 @@
       (h4 :class "font-semibold text-stone-700 mt-6 mb-2" "Phase 5 — Integration and iteration")
       (p "Write the " (code "CLAUDE.md") " protocol. Run real tasks with Claude Code — both reading and editing. Observe which comprehension tools Claude actually reaches for. Observe where it still makes structural errors. Iterate on output formats and add any missing tools. The output formats deserve careful design based on observed behaviour, not just on what seems reasonable in advance."))
 
+    ;; -----------------------------------------------------------------
+    (~docs/section :title "Try it" :id "try-it"
+      (p "Paste or edit SX source below. The tree view shows every node with its path — click a node to select it, then switch to context view to see the enclosing chain.")
+      (~sx-tools/tree-editor))
+
     ;; -----------------------------------------------------------------
     (~docs/section :title "What changes" :id "what-changes"
       (p "With SX Tools, the debugging session that found the home-stepper bug would not have happened. The workflow would have been:")