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/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"))