A resource can name the version of the build it uses

novox/hq 04-ISSUES/142, and the second of the two things ADR 0141's own
insight named: "a version cannot reach the path". A component is unpacked
into a directory named for its version so it can read its own version from
its path — and an archive named a fixed path in the manifest with nothing
interpolating the build into it, so nothing could ask for
.../versions/<version>/ and every machine took a hand-placed fallback.

A resource using an archive or a bundle may now say ${version} in any of
its values. No artifact name in the reference: the resource already says
which artifact it is for, and a second name is a second thing to keep in
step.

The version is the artifact's digest, short, and not the commit. Two
builds of one commit are meant to be the same bytes — every toolchain here
is -trimpath for that reason — so a content-addressed version means an
unchanged build resolves to the path it already had. A commit-named path
would move for an identical binary and recreate everything that reads it.

An image is refused one, with a reason: an image is not unpacked, so it
has no versioned place. Left alone it would reach a machine as literal
text and be created as a directory called ${version}.

internal/catalogue/build.go

@@ -94,12 +94,43 @@ func (m Manifest) Resolve(built []Built) (Manifest, error) {
 				m.Module, r["id"], named)
 		case ArtifactImage, ArtifactUpstream:
 			filled["image"] = artifact.Reference
+			// An image is not unpacked anywhere, so it has no directory to be named for its
+			// version and `${version}` has nothing to mean. Refused rather than left as literal
+			// text in a path, which is how it would reach a machine and be created as a directory
+			// called `${version}`.
+			for key, value := range filled {
+				if text, isText := value.(string); isText && strings.Contains(text, versionRef) {
+					return Manifest{}, fmt.Errorf(
+						"%s: %v says %s in %q, and %q is an image — an image is not unpacked, so "+
+							"it has no versioned place. %s is for an archive or a bundle",
+						m.Module, r["id"], versionRef, key, named, versionRef)
+				}
+			}
 		case ArtifactArchive, ArtifactBundle:
 			// The same on the wire: both are bytes fetched by digest and unpacked. They differ in
 			// how they were made — one packed as it stood, the other compiled first — and a
 			// machine has no reason to care which.
 			filled["source"] = artifact.Reference
 			filled["digest"] = artifact.Digest
+			// **And `${version}`, so a resource can name a place that is this build's alone**
+			// (novox/hq ADR 0141, 04-ISSUES/142). A component is unpacked into a directory named
+			// for its version so it can read its own version from its path — and until this,
+			// nothing could compose that path: an archive named a fixed one in the manifest and
+			// nothing interpolated the build into it, so nothing could ask for
+			// `…/versions/<version>/` and every machine took a hand-placed fallback.
+			//
+			// The version is the artifact's own digest, short. Not the commit: two builds of one
+			// commit are meant to be the same bytes (the toolchains are `-trimpath` for this), and
+			// a content-addressed version means an unchanged build resolves to the path it already
+			// had — so re-composing a declaration moves nothing, where a commit would move the
+			// path of an identical binary and recreate everything that reads it.
+			for key, value := range filled {
+				text, isText := value.(string)
+				if !isText || !strings.Contains(text, versionRef) {
+					continue
+				}
+				filled[key] = strings.ReplaceAll(text, versionRef, versionOf(artifact.Digest))
+			}
 		default:
 			return Manifest{}, fmt.Errorf("%s: %q is a %q, and an artifact is %q, %q, %q or %q",
 				m.Module, named, artifact.Kind, ArtifactImage, ArtifactArchive, ArtifactUpstream,
@@ -259,3 +290,28 @@ func compilesToABinary(language string) bool {
 		return false
 	}
 }
+
+// versionRef is how a resource names the version of the artifact it uses: ${version}.
+//
+// No artifact name in it, because the resource already says which artifact it is for — a second
+// name would be a second thing to keep in step with the first.
+const versionRef = "${version}"
+
+// versionOf is an artifact's version as a path names it: its digest, short.
+//
+// **Content-addressed on purpose.** The alternative is the commit a build came from, and two builds
+// of one commit are meant to produce the same bytes — every toolchain here is `-trimpath` for that
+// reason. A commit-named path would move for an identical binary, and everything reading that path
+// would be recreated for a change that is not one. A digest-named path moves exactly when the bytes
+// do.
+//
+// Twelve hex characters: enough that two of this mesh's builds will not collide, short enough to
+// read in a path and in a journal line. The `sha256:` prefix goes, because a directory name carrying
+// a colon is a directory name people quote wrong.
+func versionOf(digest string) string {
+	hex := strings.TrimPrefix(strings.TrimSpace(digest), "sha256:")
+	if len(hex) > 12 {
+		return hex[:12]
+	}
+	return hex
+}