Prerequisite reachability filters

**Scope Audited**
- Module wiki visibility config and docs.
- Toolkit wiki expression engine and visibility tests.
- Feat prerequisite rendering/reference behavior.

**Changes Made**
- Added `hidden_ref(Field, "dataset")` and `all_hidden_refs(..., "dataset")` helpers in [wiki_tables.go](/home/vicky/Projects/nwnee-shadowsoverwestgate/toolkit/internal/topdata/wiki_tables.go:914).
- Added feat visibility exclusions in [visibility.yaml](/home/vicky/Projects/nwnee-shadowsoverwestgate/module/topdata/wiki/visibility.yaml:44) for:
  - `PREREQFEAT1`
  - `PREREQFEAT2`
  - `REQSKILL`
  - `REQSKILL2`
  - `OrReqFeat0..OrReqFeat4`
- Documented the helpers in [README.md](/home/vicky/Projects/nwnee-shadowsoverwestgate/module/topdata/wiki/README.md:53).
- Added coverage in [wiki_native_test.go](/home/vicky/Projects/nwnee-shadowsoverwestgate/toolkit/internal/topdata/wiki_native_test.go:646).

**Configuration/Schema Impact**
- No schema file change. `visibility.yaml` can now use the two new expression helpers.

**Compatibility Impact**
- Existing visibility expressions are unchanged.
- Missing or `****` prerequisite fields are ignored.
- Present references that resolve to hidden or missing wiki rows are treated as unreachable.

**Tests/Validation**
- Confirmed the new test failed before implementation on unknown helper.
- Ran `go test ./internal/topdata -run TestWikiVisibility`.
- Ran `go test ./internal/topdata`.
- Rebuilt toolkit with `./build-tool.sh`.
- Ran module wiki build with local toolkit:
  `SOW_TOOLS_DEV_BINARY=../toolkit/tools/sow-toolkit ./build-wiki.sh`
  Result: `wiki pages: 771`.

internal/topdata/wiki_native_test.go

@@ -643,6 +643,90 @@ datasets:
 	}
 }
 
+func TestWikiVisibilityCanExcludeUnreachableFeatPrerequisites(t *testing.T) {
+	policy, err := parseWikiVisibilityDefinitions([]byte(`
+datasets:
+  feat:
+    include:
+      - when: "{{true}}"
+    exclude:
+      - when: "{{hidden_ref(PREREQFEAT1, \"feat\")}}"
+      - when: "{{hidden_ref(REQSKILL, \"skills\")}}"
+      - when: "{{all_hidden_refs(OrReqFeat0, OrReqFeat1, OrReqFeat2, OrReqFeat3, OrReqFeat4, \"feat\")}}"
+`), "visibility.yaml")
+	if err != nil {
+		t.Fatalf("parse visibility policy: %v", err)
+	}
+	ctx := &wikiContext{
+		visibilityPolicy: &policy,
+		visibleWikiKeys: map[string]bool{
+			"feat:visible":   true,
+			"feat:hidden":    false,
+			"skills:visible": true,
+			"skills:hidden":  false,
+		},
+		featIDToKey:  map[int]string{1: "feat:visible", 2: "feat:hidden"},
+		skillIDToKey: map[int]string{1: "skills:visible", 2: "skills:hidden"},
+	}
+	definition := policy.Datasets["feat"]
+	featRow := func(values map[string]any) map[string]any {
+		row := map[string]any{"FEAT": "Test Feat", "DESCRIPTION": "Description"}
+		for key, value := range values {
+			row[key] = value
+		}
+		return row
+	}
+	for name, tt := range map[string]struct {
+		row  map[string]any
+		want bool
+	}{
+		"visible required feat": {
+			row:  featRow(map[string]any{"PREREQFEAT1": 1}),
+			want: true,
+		},
+		"hidden required feat": {
+			row:  featRow(map[string]any{"PREREQFEAT1": 2}),
+			want: false,
+		},
+		"missing required feat is unreachable": {
+			row:  featRow(map[string]any{"PREREQFEAT1": 999}),
+			want: false,
+		},
+		"visible required skill": {
+			row:  featRow(map[string]any{"REQSKILL": "skills:visible"}),
+			want: true,
+		},
+		"hidden required skill": {
+			row:  featRow(map[string]any{"REQSKILL": "skills:hidden"}),
+			want: false,
+		},
+		"null prerequisite fields are ignored": {
+			row:  featRow(map[string]any{"PREREQFEAT1": "****", "REQSKILL": "****"}),
+			want: true,
+		},
+		"or group with visible option remains reachable": {
+			row:  featRow(map[string]any{"OrReqFeat0": 2, "OrReqFeat1": 1}),
+			want: true,
+		},
+		"or group with only hidden options is unreachable": {
+			row:  featRow(map[string]any{"OrReqFeat0": 2, "OrReqFeat1": "feat:missing"}),
+			want: false,
+		},
+		"empty or group is ignored": {
+			row:  featRow(map[string]any{"OrReqFeat0": "****", "OrReqFeat1": "****"}),
+			want: true,
+		},
+	} {
+		got, err := ctx.evaluateWikiVisibility("feat", "feat:"+name, tt.row, definition, nil)
+		if err != nil {
+			t.Fatalf("%s visibility evaluation failed: %v", name, err)
+		}
+		if got != tt.want {
+			t.Fatalf("%s expected visibility %v, got %v", name, tt.want, got)
+		}
+	}
+}
+
 func TestWikiVisibilityMetadataOverrideCannotBypassEligibility(t *testing.T) {
 	policy, err := parseWikiVisibilityDefinitions([]byte(`
 datasets: