fix(gengo): hoist every BEGINDUMP-supplied import, not just fmt/strings

doGenerate's deferred-imports patch handled only "fmt" and "strings"
— enough for inline RTL but a dead end for #pragma BEGINDUMP blocks
that import third-party / private packages (e.g. solmade's
internal/dartapi). hoistGoImports() registered every imported path
in g.imports correctly, but the final placeholder replacement never
emitted them, so the generated .go file's import block stayed
incomplete and `go build` failed with "undefined: dartapi".

Now every entry of g.imports that isn't already in the header gets
appended to the DEFERRED_IMPORTS substitution. Sorted output for
stable diffs.

Verified: a 4-line BEGINDUMP block in /tmp/poc_dartapi.prg that
imports "encoding/json" + "gitea.fivego.org/kwon_ai/solmade/internal/dartapi"
now compiles into a single 23 MB binary that calls
dartapi.AllAliases() at run time and returns the real count (3).
Full Five regression: go test compiler/, compat 56/56, std.ch 17/17,
FRB 7/7 all pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

compiler/gengo/gengo.go

@@ -180,14 +180,29 @@ func doGenerate(file *ast.File, debug, library bool) string {
 	}
 	g.emitSymCache()
 
-	// Patch deferred imports: inline RTL may add "fmt"/"strings" after header was emitted.
+	// Patch deferred imports: emitDecl may add to g.imports after the
+	// header was written. The common cases are inline RTL pulling in
+	// "fmt"/"strings", but #pragma BEGINDUMP blocks can also hoist
+	// third-party / private modules (e.g. solmade's internal/dartapi).
+	// Dump every import not already in the header so external Go
+	// packages used inside BEGINDUMP actually link.
 	result := g.buf.String()
-	var deferred string
-	if g.imports["fmt"] && !strings.Contains(result, "\"fmt\"") {
-		deferred += "\t\"fmt\"\n"
+	var deferredLines []string
+	for imp := range g.imports {
+		if strings.Contains(result, "\""+imp+"\"") {
+			continue // already emitted in the initial import block
 		}
-	if g.imports["strings"] && !strings.Contains(result, "\"strings\"") {
-		deferred += "\t\"strings\"\n"
+		if alias, ok := g.importAlias[imp]; ok && alias != "" {
+			deferredLines = append(deferredLines, fmt.Sprintf("\t%s %q", alias, imp))
+		} else {
+			deferredLines = append(deferredLines, fmt.Sprintf("\t%q", imp))
+		}
+	}
+	// Sorted for deterministic diffs.
+	sort.Strings(deferredLines)
+	deferred := strings.Join(deferredLines, "\n")
+	if deferred != "" {
+		deferred += "\n"
 	}
 	result = strings.Replace(result, "\t/*DEFERRED_IMPORTS*/\n", deferred, 1)
 	// Patch guards