User-facing docs: --commands-md flag, doc/user/ tree, anonymised script.

- `essim --commands-md [file]` instantiates the Tui, calls
  `Tui::DumpCommandsMd(ostream&)` which iterates the live registry and
  emits Markdown grouped by interactive/other, then exits. Single
  source of truth: a new `CommandSpec` field surfaces automatically.
- CMake `doc` target now `DEPENDS essim` and chains:
    doxygen → gen_api_md.py → doc/api/
    essim --commands-md      → doc/user/commands.md
- `doc/user/` adds:
    - index.md (hand-written) — first session, interactive-screen
      conventions, save/restore/replay overview.
    - scripting.md (hand-written) — `set`/`$var` expansion semantics,
      `source` event-paced execution, script-save denylist, worked
      example pointing at test/system.essim.
    - commands.md (auto-generated, regenerated by the `doc` target).
- Top-level README refocused on quick start; pointers to the new
  doc tree (user/, api/, DESIGN.md) instead of an inline command table.
- doc/README.md and DESIGN.md document the two-pipeline doc workflow.
- `test/system.essim` and user docs anonymised: bkp → backplane,
  vdn1/2/3 → payload1/2/3, cb3p → payload4, bpb/cob/ssu →
  peripheral1/2/3; netlist file names + variable names + paths all
  replaced with generic equivalents.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/tui/tui.hpp

@@ -4,6 +4,7 @@
 #include <atomic>
 #include <deque>
 #include <functional>
+#include <iosfwd>
 #include <map>
 #include <memory>
 #include <string>
@@ -122,6 +123,7 @@ public:
     Tui();
     ~Tui();
     void Run();
+    void DumpCommandsMd(std::ostream &out) const;
 
 private:
     // Lifecycle (commands.cpp)