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/plugin.toml

@@ -0,0 +1,30 @@
+[plugin]
+id = "com.example.plugin-template-v2"
+name = "Plugin Template v2"
+version = "0.1.0"
+description = "Demonstrates read/write object round-trips with the host."
+author = "Your Name"
+license = "GPL-3.0-only"
+repository = "your-account/your-plugin-repo"
+
+[opencad]
+api_version = 5
+# Both filled at release-build time from the actual compiler and Cargo.lock's
+# resolved `acadrust` source — see docs/plugin-template's release.yml for the
+# exact substitution. Do not fill these in by hand. Declare acadrust_source
+# even though it's optional: the host's compatibility check only runs at all
+# if this field is present, so an omitted field is silently treated as
+# compatible rather than rejected (see plugin-architecture.md's "Build-time
+# ABI metadata"). This template ships without its own CI workflow (see
+# DESIGN.md) — copy docs/plugin-template's release.yml if you turn this into
+# a real, released plugin.
+rustc_version = "__RUSTC_VERSION__"
+acadrust_source = "__ACADRUST_SOURCE__"
+ribbon_order = 100
+command_prefixes = [
+    "COUNT_SURVEY_POINTS",
+    "ADD_SURVEY_POINT",
+    "MARK_FIRST_SURVEY_POINT",
+    "COUNT_MARKED_SURVEY_POINTS",
+]
+xdata_apps = ["SURVEYMARK"]