MCP: prompts, resource templates, completions and logging

The server answered tools/* and two fixed resources and nothing else, and
resources/templates/list returned an empty array. This adds the rest of the surface a
client can actually use, against 2025-06-18.

- prompts/list and prompts/get: five workflows (poster, icon-set, recolor,
  trace-and-style, export-set) with templated arguments. A missing required argument
  is -32602 and names itself.
- resources/templates/list, and resources/read for the templates: object/{id},
  command/{id}, effect/{id}, swatch/{name}. Reading one thing at a time instead of
  vectorcraft://document/json, which on a real document is thousands of lines an agent
  pays for again on every change.
- completion/complete for prompt arguments and template variables, ranked prefix
  first, capped at 100 with total and hasMore. The values are read live, so a suggested
  format, effect id, command id, trace preset, swatch or object id is one the engine
  has.
- logging/setLevel and notifications/message. Nothing installed a log::Log before, so
  the engine's log::debug! calls went nowhere; this is the sink, silent until asked.
  Installing it is vectorcraft-cli's job rather than the library's, so an embedder
  that already has a logger keeps it.

docs/mcp.md gains a Protocol section with the capability table and a Not implemented
section: subscriptions, progress, cancellation, sampling, elicitation and Streamable
HTTP all need the transport to interleave reads, which this one does not — handle_line
answers a line with a reply, so it neither reads an incoming cancellation nor can write
progress before the reply it belongs to.

62 tests (50 unit, 12 property/fuzz). protocol_props.rs gained five: prompts survive
junk arguments, completion/complete never panics and always answers at most 100
string values with a consistent hasMore, every template reads or fails cleanly, and
prompts/completions/templates are swept with junk params while asserting the document
comes back byte-identical.

ROADMAP: one bullet under Automation in Shipped so far and the changelog line. The
parity table and the gap list are untouched — MCP is agent automation, so feature
parity does not move.

apps/vectorcraft-cli/src/main.rs

@@ -142,6 +142,9 @@ fn mcp(args: &[String]) -> Result<(), String> {
         }
     };
     eprintln!("vectorcraft-cli: MCP server on stdio ({})", backend.describe());
+    // The binary owns the logger, not the library: installing one here keeps an embedder that
+    // uses `vectorcraft_mcp` free to bring its own. Silent until a client sends logging/setLevel.
+    vectorcraft_mcp::logging::install();
     let stdin = std::io::stdin();
     let stdout = std::io::stdout();
     Server::new(backend).serve(stdin.lock(), stdout.lock()).map_err(|e| e.to_string())