Leave out a block its holder cannot render, and keep if-capability to known names on offered kinds (hq ADR 0255)

A piece whose shows the holder's template does not know rendered as nothing and failed
the whole machine's declaration; it is now left out and named, for push, plan and the
merge gate. quote escapes DEL, which TOML refuses bare. An if-capability nothing
detects, or on a kind a holder depends on, would drop a piece silently, so both are
refused.

cmd/mesh-controller/sendable.go

@@ -58,6 +58,9 @@ type sendable struct {
 	// make (Foreseeing, novox/hq issue 275). Never on the wire, and a declaration that has any is never
 	// sent.
 	foreseen []string
+	// unplaced is every contribution its holder's template could not render, naming its module and
+	// why (novox/hq ADR 0255): left out of this declaration, for push and plan to say, never on the wire.
+	unplaced []string
 	// Builds is the build of each module this declaration carries — module to the commit its build
 	// was made from — recorded with the send and never on the wire (novox/hq issue 259, ADR 0221).
 	// Composed only on the send path; nil records that it is not known.