Catch up on merges the bus announced and never handed over (hq issue 266)

The controller acted only on what its events consumer handed it, so a merge
the bus skipped left modules behind with nothing said. The stream is now read
back every five minutes on a single-filter consumer, and any merge that would
still move a module after ten minutes is said and acted on.

cmd/mesh-controller/upgrades.go

@@ -8,6 +8,7 @@ import (
 	"path"
 	"regexp"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/novox/mesh-controller/internal/catalogue"
@@ -266,6 +267,11 @@ func notNow(err error) error {
 // (novox/hq 04-ISSUES/131). Nothing is pushed here: what a finished build does to the machines
 // running the module is the upgrade's decision, taken when the catalogue announces it.
 func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
+	// One merge acted on at a time, whoever hands it over: the bus, or the catch-up that reads back
+	// what the bus did not hand over (novox/hq issue 266). Each judges against what the other wrote.
+	actingOnMerges.Lock()
+	defer actingOnMerges.Unlock()
+
 	inv := f.open.inventory
 	entries, err := inv.Catalogued(ctx)
 	if err != nil {
@@ -276,34 +282,7 @@ func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
 		return notNow(err)
 	}
 
-	// Two kinds of module are affected by one merge, and they are affected differently.
-	//
-	// A module **built from** this repository and branch has moved: the mesh records the new commit
-	// as what its source now has, and only what the merge actually changed is rebuilt. A module that
-	// only **packages source from** it has not moved — its own source is somewhere else, at the
-	// commit it already records — so it is rebuilt and its record left alone. Writing this commit as
-	// its source would make it permanently behind a repository its manifest does not come from.
-	var from, packaging []inventory.Entry
-	already := 0
-	for _, e := range entries {
-		switch {
-		case sourceIs(e.Source, m):
-			if e.Source.BuiltFrom == m.Commit {
-				already++
-				continue
-			}
-			// **A merge older than the last look at the source is history, not a move.** The forge
-			// announces what it finds merged, and an old merge surfacing late would otherwise move
-			// the recorded head backwards and rebuild everything built from that repository, once
-			// per old merge (2026-09-28).
-			if isHistory(m.MergedAt, e.Source.Seen) {
-				continue
-			}
-			from = append(from, e)
-		case readsFrom(read[e.Manifest.Module], m):
-			packaging = append(packaging, e)
-		}
-	}
+	from, packaging, already := mergeCandidates(m, entries, read)
 	if len(from) == 0 && len(packaging) == 0 {
 		// "Already built from it" and "nothing reads it" are different facts, and reading the first
 		// as the second sends somebody looking for a broken trigger when the mesh is up to date.
@@ -438,6 +417,57 @@ func (f following) SourceMoved(ctx context.Context, m link.SourceMoved) error {
 	return nil
 }
 
+// actingOnMerges keeps one merge acted on at a time (novox/hq issue 266).
+var actingOnMerges sync.Mutex
+
+// mergeCandidates is what one merge could move, judged against what the catalogue holds.
+//
+// Two kinds of module are affected by one merge, and they are affected differently.
+//
+// A module **built from** this repository and branch has moved: the mesh records the new commit as
+// what its source now has, and only what the merge actually changed is rebuilt. A module that only
+// **packages source from** it has not moved — its own source is somewhere else, at the commit it
+// already records — so it is rebuilt and its record left alone. Writing this commit as its source
+// would make it permanently behind a repository its manifest does not come from. `already` counts
+// the modules built from this repository that are already built from this very commit.
+func mergeCandidates(m link.SourceMoved, entries []inventory.Entry,
+	read map[string][]inventory.ReadRepository) (from, packaging []inventory.Entry, already int) {
+	for _, e := range entries {
+		switch {
+		case sourceIs(e.Source, m):
+			if e.Source.BuiltFrom == m.Commit {
+				already++
+				continue
+			}
+			// **A merge older than the last look at the source is history, not a move.** The forge
+			// announces what it finds merged, and an old merge surfacing late would otherwise move
+			// the recorded head backwards and rebuild everything built from that repository, once
+			// per old merge (2026-09-28).
+			if isHistory(m.MergedAt, e.Source.Seen) {
+				continue
+			}
+			from = append(from, e)
+		case readsFrom(read[e.Manifest.Module], m):
+			packaging = append(packaging, e)
+		}
+	}
+	return from, packaging, already
+}
+
+// wouldMove is the modules built from the merged repository that acting on this merge would mark as
+// moved and rebuild — SourceMoved's judgement, made without acting (novox/hq issue 266). Empty for a
+// merge already acted on: acting marks each of them as looked at, so the merge then reads as history.
+//
+// **Only the modules built from it, never the ones that merely package source from it.** Acting
+// records nothing about those, so a merge acted on would go on reading as unacted for them, and be
+// acted on again on every look. A merge that moves both is caught by the first kind, and acting on it
+// rebuilds the second as well.
+func wouldMove(m link.SourceMoved, entries []inventory.Entry,
+	read map[string][]inventory.ReadRepository) []inventory.Entry {
+	from, _, _ := mergeCandidates(m, entries, read)
+	return whatTheMergeTouched(from, entries, m)
+}
+
 // sourceIs is whether a recorded source is the repository and branch a merge announced. A source on
 // the git seat is recorded as its path on the forge; one elsewhere as the URL it was cloned from.
 // An empty recorded ref is the repository's default branch, which is what a merge into the base