A changed file touches exactly the modules whose build reads it (hq issue 280, ADR 0237)

The builder reads a module's own directory (the repository for one built from its root)
and a repository its recipe packages, nothing else. A file in no module's directory was
read as shared code and rebuilt everything built from the repository: 103 modules for a
merge-check.sh added at the catalogue's root. It now touches nothing, in the merge
handler, the release planner and the pull request's check alike, and the gate says so.

internal/link/events.go

@@ -223,8 +223,8 @@ type PullUpdated struct {
 	PathsTruncated bool     `json:"paths_truncated,omitempty"`
 	// ModuleDirs are the directories above the changed files that hold a `module.json` at the head, read
 	// as the merge announcer reads them at a merge commit (issue 278); ModuleDirsSaid says it looked.
-	// What the controller maps a pull request onto the mesh's module graph with: a file inside one is
-	// that module's, held or not, and one inside none, in a repository modules are built from, is shared.
+	// What the controller finds a module the graph does not hold yet with: a directory the change adds a
+	// module.json in is a new module, checked before it merges.
 	ModuleDirs     []string `json:"module_dirs,omitempty"`
 	ModuleDirsSaid bool     `json:"module_dirs_said,omitempty"`
 	// MergeCheck says the head holds a merge-check.sh at its root — the repository's own tests, the