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
 		}
 	}