docs(32-04): grobotstxt decision note

- github.com/jimsmart/grobotstxt v1.0.3 for the robots.txt tester and
  the noindex-blocked warning in sm-seo-plugin (D-99), plugin-only
- STACK.md Supporting Libraries row

.planning/notes/2026-10-27-robots-txt-grobotstxt.md

@@ -0,0 +1,33 @@
+# Decision: the robots.txt tester uses grobotstxt
+
+**Date:** 2026-10-27
+**Status:** Accepted
+**Context:** Phase 32 (SEO foundation, D-22, D-24, D-99), 32-RESEARCH.md Standard Stack and Package Legitimacy Audit, plan 32-04 Task 4
+
+## Decision
+
+The robots.txt tester of `sm-seo-plugin` (element E5, widget action `robots_test`) and the check that warns when robots.txt blocks a noindex address (D-24) match URLs with `github.com/jimsmart/grobotstxt`.
+
+- **`github.com/jimsmart/grobotstxt` pinned at v1.0.3.** Published 2022-03-16 (proxy.golang.org), Apache-2.0, origin `github.com/jimsmart/grobotstxt` at tag `v1.0.3`. It is the newest tag.
+- **Why this library.** It is a function-for-function Go port of Google's C++ robots.txt parser and matcher (`google/robotstxt`), so the tester answers what Googlebot does:
+  - the longest matching rule wins, and `Allow` wins a tie with `Disallow`;
+  - `*` matches any run of characters and `$` anchors the end;
+  - user-agent groups are chosen as Google chooses them (the most specific group, else `*`);
+  - `RobotsMatcher.MatchingLine()` gives the line number of the deciding rule (0 when no rule matched), which E5 shows to the editor.
+- **What the plugin calls:** `NewRobotsMatcher()`, `(*RobotsMatcher).AgentAllowed(robotsBody, userAgent, uri)` and `MatchingLine()`. Nothing else, and only in `sm-seo-plugin/classes/robots.go`.
+- **Plugin-only.** Only `sm-seo-plugin` requires it. The framework `go.mod` does not, and `grep jimsmart/grobotstxt go.mod` in summercms.go stays empty.
+- **Imported packages:** the library imports the standard library only (`go list -deps` on a scratch module lists `github.com/jimsmart/grobotstxt` as its single non-stdlib package). Its go.mod requires ginkgo and gomega for its own tests; they reach a consumer's go.sum and are never compiled into a binary.
+
+## Rejected alternatives
+
+- **`github.com/temoto/robotstxt` v1.1.2** (2021, MIT). Older, with its own parser, no matching-line API, and no documented Google longest-match semantics.
+- **A hand-written RFC 9309 matcher** (about 200 lines). Standard library first and no stale dependency, but protocol code the project prefers to take from a maintained library, and Google's behaviour has edge cases the port already covers. RFC 9309 is frozen (2022), so a 2022 release of a faithful port is not a staleness risk in practice.
+
+## Package legitimacy
+
+The package-legitimacy seam cannot check Go modules, so grobotstxt was gated by a blocking human check (plan 32-03 Task 4) and approved by Jakub before any `go get`. The orchestrator verified that v1.0.3 resolves through proxy.golang.org; the licence file in the module is Apache-2.0.
+
+## Consequences
+
+- `sm-seo-plugin/go.mod` requires `github.com/jimsmart/grobotstxt v1.0.3`; applications that mount the plugin get it through the plugin.
+- If the library ever needs replacing, the tester's answer shape (allowed or blocked, the line and its text, blocked by a bot switch) is the plugin's own, so only `classes/robots.go` changes.