One reader for a declaration file, because there were three

Found raising two machines: `apply <file>` refused the bundle example in
this repository with `invalid character '/'`. The bundle strips whole-line
comments; apply handed the raw bytes to the parser. So a file this repo
ships could be built into a binary and not applied from disk.

This is the third instance of one fault. There is already a test here
named "what validates is what is applied", written when `mesh-host
bundle` said yes and `reconcile` said no about the same artefact — two
paths to one thing, disagreeing. Fixing that instance left the shape
intact, so it came back somewhere else.

So the fix is structural rather than local: `declaration.ParseFileTrusted`
is the one way to read a declaration from disk, and the bundle and apply
both use it. Comment handling and its test now live in one place, since
having them in two is how it came to be done in two.

The wire format is untouched — over the link it stays exactly JSON,
because a format with a second thing to strip is a format with a second
thing to disagree about. Asserted, and confirmed to fail if the link
starts stripping.

internal/declaration/declaration.go

@@ -521,3 +521,36 @@ func vocabulary() string {
 	sort.Strings(names)
 	return strings.Join(names, ", ")
 }
+
+// ParseFileTrusted reads a declaration from a file somebody handed this host.
+//
+// The same as ParseTrusted, and it allows whole-line `//` comments first. A pinned, hand-authored
+// artefact that nobody can annotate is one nobody can review — the substrate bundle is mostly
+// explanation of why each digest is what it is.
+//
+// **Only for a file, never for the link.** Over the link the format stays exactly JSON, because
+// a wire format with a second thing to strip is a wire format with a second thing to disagree
+// about.
+//
+// It exists because there were two readers for one file: the bundle stripped comments and `apply`
+// did not, so the example bundle in this repository could be built into a binary and not applied
+// from disk. The failure was `invalid character '/'`, which names the symptom and not the cause.
+func ParseFileTrusted(raw []byte) (*Declaration, error) {
+	return ParseTrusted(stripComments(raw))
+}
+
+// stripComments removes whole lines beginning with `//`.
+//
+// Only whole lines: anything cleverer would need to know where strings begin and end, and a
+// parser that half-understands its input is worse than one that does not try. A `//` inside a
+// value — every image reference has one — is untouched.
+func stripComments(raw []byte) []byte {
+	var kept []string
+	for _, line := range strings.Split(string(raw), "\n") {
+		if strings.HasPrefix(strings.TrimSpace(line), "//") {
+			continue
+		}
+		kept = append(kept, line)
+	}
+	return []byte(strings.Join(kept, "\n"))
+}