Read a rebuild of an unchanged source as no move, whatever image digest it made (hq issue 280)

An image is not byte-reproducible, so ADR 0236's 'same artifacts is no move'
never held for one: a catalogue merge that did not touch the bus rebuilt it,
and every send to the control node waited for a planned bus upgrade.

The builder now records a source fingerprint per build (module tree, context
trees, bases and toolchains by digest). A rebuild with the fingerprint of the
build it repeats is registered with that build's artifacts, handed to modules
standing on it, holds no push, demands no bus step, and a plan sends and
gates nothing for it. Identical artifacts remain a second way to be no move.

internal/link/build.go

@@ -179,6 +179,13 @@ type BuildResult struct {
 	// manifest the mesh keeps says nothing about it (novox/hq 04-ISSUES/131).
 	Read []ReadRepository `json:"read,omitempty"`
 
+	// SourceFingerprint is what the build was made from, hashed (novox/hq issue 280): the module's
+	// tree at the commit, the trees of the contexts it read, its bases and toolchains by digest. Two
+	// builds with one fingerprint are one build, however their digests differ — an image is not
+	// byte-reproducible. Empty from a builder that predates it, or where the source does not pin the
+	// build; then only identical artifacts make a rebuild no move.
+	SourceFingerprint string `json:"source-fingerprint,omitempty"`
+
 	// Failed is why, when it did.
 	Failed string `json:"failed,omitempty"`