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/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)