feat: extract set_fields rule forces Int fields on gff_json merge targets (#40)

Adds a `set_fields` option to `extract.merge.gff_json` rules: forces existing Int fields to a fixed value on every extract, applied after preserve_fields/merge_lists so the forced value always wins. Fields absent from the document stay absent. Rule targets can now be globs (e.g. `areas/*.are.json`).

Motivation: sow-module area weather chances must stay 0 (scripted regional weather owns weather); this makes the extract pipeline enforce it instead of a post-extract script.

Covered by `TestExtractSetsConfiguredGFFJSONFields`; `make check` passes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #40
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

internal/pipeline/extract.go

@@ -350,26 +350,48 @@ func mergeExtractedGFFJSON(p *project.Project, target string, extracted *gff.Doc
 	}
 
 	raw, err := os.ReadFile(target)
-	if err != nil {
-		if errors.Is(err, os.ErrNotExist) {
-			return nil
-		}
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		return fmt.Errorf("read existing source %s: %w", target, err)
 	}
-	var existing gff.Document
-	if err := json.Unmarshal(raw, &existing); err != nil {
-		return fmt.Errorf("parse existing source %s: %w", target, err)
+	if err == nil {
+		var existing gff.Document
+		if err := json.Unmarshal(raw, &existing); err != nil {
+			return fmt.Errorf("parse existing source %s: %w", target, err)
+		}
+
+		for _, label := range rule.PreserveFields {
+			if field, ok := gffField(existing.Root, label); ok {
+				setGFFField(&extracted.Root, field)
+			}
+		}
+		for _, listRule := range rule.MergeLists {
+			if err := mergeGFFListByKey(&extracted.Root, existing.Root, listRule); err != nil {
+				return err
+			}
+		}
 	}
 
-	for _, label := range rule.PreserveFields {
-		if field, ok := gffField(existing.Root, label); ok {
-			setGFFField(&extracted.Root, field)
+	// set_fields runs last so a forced value also wins over preserve_fields.
+	for _, setRule := range rule.SetFields {
+		if err := setGFFIntField(&extracted.Root, setRule); err != nil {
+			return fmt.Errorf("set_fields on %s: %w", target, err)
 		}
 	}
-	for _, listRule := range rule.MergeLists {
-		if err := mergeGFFListByKey(&extracted.Root, existing.Root, listRule); err != nil {
-			return err
+	return nil
+}
+
+// setGFFIntField forces an existing Int field to a fixed value; a field the
+// document does not have is left absent rather than invented.
+func setGFFIntField(s *gff.Struct, rule project.ExtractSetFieldRule) error {
+	for index, field := range s.Fields {
+		if field.Label != rule.Field {
+			continue
 		}
+		if _, ok := field.Value.(gff.IntValue); !ok {
+			return fmt.Errorf("field %q is %s, not Int", rule.Field, field.Type)
+		}
+		s.Fields[index].Value = gff.IntValue(rule.Value)
+		return nil
 	}
 	return nil
 }
@@ -384,7 +406,7 @@ func extractGFFJSONMergeRule(p *project.Project, target string) (project.Extract
 		return project.ExtractGFFJSONMergeRule{}, false, nil
 	}
 	for _, rule := range p.EffectiveConfig().Extract.Merge.GFFJSON {
-		if rule.Target == rel {
+		if rule.Target == rel || matchPathPattern(rel, rule.Target) {
 			return rule, true, nil
 		}
 	}

internal/pipeline/pipeline_test.go

@@ -3503,6 +3503,100 @@ extract:
 	}
 }
 
+func TestExtractSetsConfiguredGFFJSONFields(t *testing.T) {
+	root := t.TempDir()
+	mustMkdir(t, filepath.Join(root, "src", "module"))
+	mustMkdir(t, filepath.Join(root, "src", "areas"))
+	mustMkdir(t, filepath.Join(root, "assets"))
+	mustMkdir(t, filepath.Join(root, "build"))
+
+	mustWriteFile(t, filepath.Join(root, "nwn-tool.yaml"), `
+module:
+  name: Test Module
+  resref: testmod
+paths:
+  source: src
+  assets: assets
+  build: build
+extract:
+  merge:
+    gff_json:
+      - target: areas/*.are.json
+        set_fields:
+          - field: ChanceRain
+            value: 0
+          - field: ChanceSnow
+            value: 0
+`)
+
+	mustWriteFile(t, filepath.Join(root, "src", "module", "module.ifo.json"), `{
+  "file_type": "IFO ",
+  "file_version": "V3.2",
+  "root": {
+    "struct_type": 0,
+    "fields": [
+      {
+        "label": "Mod_Name",
+        "type": "CExoString",
+        "value": "Test Module"
+      }
+    ]
+  }
+}
+`)
+	mustWriteFile(t, filepath.Join(root, "src", "areas", "area_a.are.json"), `{
+  "file_type": "ARE ",
+  "file_version": "V3.2",
+  "root": {
+    "struct_type": 0,
+    "fields": [
+      {
+        "label": "ChanceRain",
+        "type": "Int",
+        "value": 40
+      },
+      {
+        "label": "WindPower",
+        "type": "Int",
+        "value": 2
+      }
+    ]
+  }
+}
+`)
+
+	p, err := project.Load(root)
+	if err != nil {
+		t.Fatalf("load project: %v", err)
+	}
+	if err := p.ValidateLayout(); err != nil {
+		t.Fatalf("validate layout: %v", err)
+	}
+	if err := p.Scan(); err != nil {
+		t.Fatalf("scan: %v", err)
+	}
+	if _, err := BuildModule(p); err != nil {
+		t.Fatalf("build module: %v", err)
+	}
+	if err := p.Scan(); err != nil {
+		t.Fatalf("rescan before extract: %v", err)
+	}
+	if _, err := Extract(p); err != nil {
+		t.Fatalf("extract: %v", err)
+	}
+
+	document := readGFFJSON(t, filepath.Join(root, "src", "areas", "area_a.are.json"))
+	if got, want := fieldValue(t, document.Root, "ChanceRain"), gff.IntValue(0); got != want {
+		t.Fatalf("expected forced ChanceRain %#v, got %#v", want, got)
+	}
+	if got, want := fieldValue(t, document.Root, "WindPower"), gff.IntValue(2); got != want {
+		t.Fatalf("expected untouched WindPower %#v, got %#v", want, got)
+	}
+	if _, ok := gffField(document.Root, "ChanceSnow"); ok {
+		t.Fatalf("expected absent ChanceSnow to stay absent")
+	}
+}
+
 func TestExtractNormalizesResourceNamesToLowercase(t *testing.T) {
 	root := t.TempDir()
 	mustMkdir(t, filepath.Join(root, "src", "blueprints", "items"))

internal/project/project.go

@@ -281,6 +281,15 @@ type ExtractGFFJSONMergeRule struct {
 	Target         string                 `json:"target" yaml:"target"`
 	PreserveFields []string               `json:"preserve_fields" yaml:"preserve_fields"`
 	MergeLists     []ExtractListMergeRule `json:"merge_lists" yaml:"merge_lists"`
+	SetFields      []ExtractSetFieldRule  `json:"set_fields" yaml:"set_fields"`
+}
+
+// ExtractSetFieldRule forces an Int field to a fixed value on every extract,
+// overriding whatever the toolset saved. Fields absent from the extracted
+// document are left absent.
+type ExtractSetFieldRule struct {
+	Field string `json:"field" yaml:"field"`
+	Value int32  `json:"value" yaml:"value"`
 }
 
 type ExtractListMergeRule struct {
@@ -980,6 +989,12 @@ func validateExtractMergeConfig(config ExtractMergeConfig) []error {
 			seenTargets[normalized] = struct{}{}
 		}
 
+		for setIndex, setRule := range rule.SetFields {
+			if strings.TrimSpace(setRule.Field) == "" {
+				failures = append(failures, fmt.Errorf("%s.set_fields[%d].field must not be empty", prefix, setIndex))
+			}
+		}
+
 		for listIndex, listRule := range rule.MergeLists {
 			listPrefix := fmt.Sprintf("%s.merge_lists[%d]", prefix, listIndex)
 			if strings.TrimSpace(listRule.Field) == "" {
@@ -1346,6 +1361,9 @@ func normalizeConfig(cfg *Config) {
 		}
 		cfg.Extract.Merge.GFFJSON[i].Target = target
 		cfg.Extract.Merge.GFFJSON[i].PreserveFields = normalizeStringSlice(cfg.Extract.Merge.GFFJSON[i].PreserveFields)
+		for j := range cfg.Extract.Merge.GFFJSON[i].SetFields {
+			cfg.Extract.Merge.GFFJSON[i].SetFields[j].Field = strings.TrimSpace(cfg.Extract.Merge.GFFJSON[i].SetFields[j].Field)
+		}
 		for j := range cfg.Extract.Merge.GFFJSON[i].MergeLists {
 			cfg.Extract.Merge.GFFJSON[i].MergeLists[j].Field = strings.TrimSpace(cfg.Extract.Merge.GFFJSON[i].MergeLists[j].Field)
 			cfg.Extract.Merge.GFFJSON[i].MergeLists[j].KeyField = strings.TrimSpace(cfg.Extract.Merge.GFFJSON[i].MergeLists[j].KeyField)