docs(plugin): fix inaccuracies and flag real gaps found building a plugin

Found while building an out-of-tree scripting plugin from scratch against
this doc and the actual ocs_plugin_api/host source. Two categories:

Fixed outright (docs/examples didn't match the real code):
- The worked CadModule example returned Vec<RibbonGroup> from
  ribbon_groups(); the real trait requires &[RibbonGroup] - the example
  wouldn't compile. Fixed with the OnceLock pattern the host's own
  benchmark code already uses.
- Reconciled a contradiction between "process isolation removes the need
  for a shared Rust ABI" and the exact-rustc_version requirement described
  a few paragraphs earlier in the same doc.
- The roadmap's "Next" list still had external automation as unstarted;
  --mcp/--serve are real and operational (src/cli.rs, src/mcp.rs).
- Added the missing plugin.toml to docs/plugin-template-v2 (its own
  DESIGN.md already claimed one could be installed).
- Documented that plugin-template's CI floats rust-toolchain@stable, which
  doesn't guarantee matching the host's exact rustc - added acadrust_source
  to both templates' plugin.toml plus the release.yml step that fills it
  from Cargo.lock's resolved acadrust source, alongside the existing
  rustc_version fill.

Flagging for your call, not fixed here (found the behavior, not sure it's
the intended one, prefer to ask than guess at a fix):
- acadrust_compatible()'s ABI gate only runs at all if plugin.toml declares
  acadrust_source - an undeclared field is silently treated as compatible,
  unlike rustc_version (undeclared = incompatible). The same shortcut
  appears in two places: ExternalPlugin::acadrust_compatible
  (src/plugin/external.rs) and the inline check inside fetch_release_info
  (src/plugin/marketplace.rs). If this is deliberate (e.g. back-compat with
  manifests from before the field existed), no action needed beyond a
  comment; if not, both call sites need the same fix.
- The command line still uppercases typed/pasted input even with the `>`
  literal-spaces prefix (Message::CommandInput, src/app/update/mod.rs) -
  `>` only stops a space from auto-submitting the line, it doesn't skip
  `s.to_uppercase()`. A plugin command needing a case-sensitive typed
  argument has no escape today short of automation.
- fetch_release_info (src/plugin/marketplace.rs) returns every release
  under a linked repo with no filter on the manifest's own id field (which
  it already parses to build ReleaseInfo) - a repo hosting more than one
  plugin under tag-namespaced releases surfaces all of them mixed
  together in one plugin's version list.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/plugin-template-v2/DESIGN.md

@@ -64,8 +64,12 @@ From the workspace root:
 cargo build -p plugin-template-v2
 ```
 
-The resulting cdylib plus `plugin.toml` can be installed into the host's
-plugins folder (see `docs/plugin-architecture.md`).
+The resulting cdylib plus `plugin.toml` (checked in alongside this file) can
+be installed into the host's plugins folder (see `docs/plugin-architecture.md`).
+`plugin.toml`'s `rustc_version`/`acadrust_source` placeholders are filled by
+CI in `docs/plugin-template` (v1) — this template has no release workflow of
+its own, so fill them by hand (`rustc --version`, and `Cargo.lock`'s resolved
+`acadrust` package `source` line) before installing a real build.
 
 ---