feat(fnode): --go-replace and --module for BEGINDUMP-import use cases

Two CLI knobs that together let PRG #pragma BEGINDUMP blocks import
private Go modules and their internal/* packages:

  --go-replace pkg=path
    Adds `require pkg v0.0.0` + `replace pkg => path` to the temp
    go.mod, so `go mod tidy` doesn't try (and fail) to fetch a
    module that lives only on local disk or behind a private
    forge (Gitea, etc.).

  --module <name>
    Overrides the temp-build module name (default fnode_build).
    Use when the BEGINDUMP block needs to reach into another
    module's internal/ packages — Go's internal-visibility rule
    requires the importer to share a parent path with the
    importee, so the build module needs to live somewhere under
    that parent.

Worked example (solmade integration PoC):

  fnode build /tmp/poc_dartapi.prg \\
    --go-replace gitea.fivego.org/kwon_ai/solmade=/Users/charleskwon/solmade \\
    --module gitea.fivego.org/kwon_ai/solmade/_fnode_build \\
    -o poc_dart

A 4-line BEGINDUMP block now imports
"gitea.fivego.org/kwon_ai/solmade/internal/dartapi", links its
AllAliases() function, and returns the real count (3) at run time.
Single 23 MB Go binary, no Node, no FFI.

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

cmd/fnode/main.go

@@ -20,6 +20,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sort"
 	"strings"
 
 	"five/compiler/analyzer"
@@ -68,8 +69,8 @@ func printUsage() {
 	fmt.Fprintln(os.Stderr, version)
 	fmt.Fprintln(os.Stderr, "")
 	fmt.Fprintln(os.Stderr, "Usage:")
-	fmt.Fprintln(os.Stderr, "  fnode build <file.prg>... [-o out] [-I dir]... [--rtl pkg]...")
-	fmt.Fprintln(os.Stderr, "  fnode run   <file.prg>     [-I dir]... [--rtl pkg]...")
+	fmt.Fprintln(os.Stderr, "  fnode build <file.prg>... [-o out] [-I dir]... [--rtl pkg]... [--go-replace pkg=path]...")
+	fmt.Fprintln(os.Stderr, "  fnode run   <file.prg>     [-I dir]... [--rtl pkg]... [--go-replace pkg=path]...")
 	fmt.Fprintln(os.Stderr, "  fnode version")
 }
 
@@ -78,10 +79,24 @@ type buildOpts struct {
 	output      string
 	includes    []string
 	rtlPkgs     []string
+	// goReplace maps a Go module path to a local filesystem path that
+	// gets emitted as `replace <path> => <local>` in the temp go.mod.
+	// Lets PRG #pragma BEGINDUMP blocks import private modules that
+	// `go mod tidy` would otherwise fail to fetch (e.g. a sibling
+	// project living outside any module proxy).
+	goReplace map[string]string
+	// goModule overrides the default temp-build module name
+	// ("fnode_build"). Set this to a path inside an existing module
+	// when the BEGINDUMP block needs to import that module's
+	// internal/* packages — Go's internal-visibility rule only lets
+	// callers whose own module path lives under the same parent see
+	// internal/.
+	//   --module gitea.fivego.org/kwon_ai/solmade/_fnode_build
+	goModule string
 }
 
 func parseCommon(args []string) buildOpts {
-	var o buildOpts
+	o := buildOpts{goReplace: map[string]string{}}
 	for i := 0; i < len(args); i++ {
 		a := args[i]
 		switch {
@@ -96,6 +111,18 @@ func parseCommon(args []string) buildOpts {
 		case a == "--rtl" && i+1 < len(args):
 			o.rtlPkgs = append(o.rtlPkgs, args[i+1])
 			i++
+		case a == "--go-replace" && i+1 < len(args):
+			// --go-replace gitea.fivego.org/kwon_ai/solmade=/abs/path
+			spec := args[i+1]
+			i++
+			eq := strings.IndexByte(spec, '=')
+			if eq <= 0 || eq == len(spec)-1 {
+				fatal("--go-replace expects pkg=path, got: " + spec)
+			}
+			o.goReplace[spec[:eq]] = spec[eq+1:]
+		case a == "--module" && i+1 < len(args):
+			o.goModule = args[i+1]
+			i++
 		default:
 			o.prgFiles = append(o.prgFiles, a)
 		}
@@ -124,7 +151,7 @@ func cmdBuild(args []string) {
 
 	emitGeneratedSources(tmpDir, o)
 	writeRootGo(tmpDir, o.rtlPkgs)
-	writeGoMod(tmpDir)
+	writeGoMod(tmpDir, o.goReplace, o.goModule)
 
 	runGo(tmpDir, "mod", "tidy")
 	runGo(tmpDir, "build", "-o", absOut, ".")
@@ -141,7 +168,7 @@ func cmdRun(args []string) {
 
 	emitGeneratedSources(tmpDir, o)
 	writeRootGo(tmpDir, o.rtlPkgs)
-	writeGoMod(tmpDir)
+	writeGoMod(tmpDir, o.goReplace, o.goModule)
 
 	runGo(tmpDir, "mod", "tidy")
 	runGoInteractive(tmpDir, "run", ".")
@@ -288,10 +315,14 @@ func writeRootGo(tmpDir string, rtlPkgs []string) {
 	writeFile(filepath.Join(tmpDir, "fnode_rtl.go"), sb.String())
 }
 
-func writeGoMod(tmpDir string) {
+func writeGoMod(tmpDir string, extraReplace map[string]string, modName string) {
 	five := mustAbs(fiveRoot())
 	fnode := mustAbs(fnodeRoot())
-	mod := fmt.Sprintf(`module fnode_build
+	if modName == "" {
+		modName = "fnode_build"
+	}
+	var sb strings.Builder
+	fmt.Fprintf(&sb, `module %s
 
 go 1.21
 
@@ -303,8 +334,22 @@ require (
 replace five => %s
 
 replace fivenode_go => %s
-`, five, fnode)
-	writeFile(filepath.Join(tmpDir, "go.mod"), mod)
+`, modName, five, fnode)
+	// Sorted iteration for stable go.mod output.
+	keys := make([]string, 0, len(extraReplace))
+	for k := range extraReplace {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, pkg := range keys {
+		path := extraReplace[pkg]
+		if !filepath.IsAbs(path) {
+			path = mustAbs(path)
+		}
+		fmt.Fprintf(&sb, "\nrequire %s v0.0.0\n", pkg)
+		fmt.Fprintf(&sb, "replace %s => %s\n", pkg, path)
+	}
+	writeFile(filepath.Join(tmpDir, "go.mod"), sb.String())
 }
 
 func mustAbs(p string) string {