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_tables.go

@@ -911,11 +911,61 @@ func (p *wikiExprParser) callHelper(name string, args []any) (any, error) {
 			}
 		}
 		return false, nil
+	case "hidden_ref":
+		if len(args) != 2 {
+			return nil, fmt.Errorf("hidden_ref expects 2 arguments")
+		}
+		hidden, err := p.hiddenWikiReference(args[0], stringifyWikiTableValue(args[1]))
+		if err != nil {
+			return nil, err
+		}
+		return hidden, nil
+	case "all_hidden_refs":
+		if len(args) < 2 {
+			return nil, fmt.Errorf("all_hidden_refs expects at least 2 arguments")
+		}
+		targetDataset := stringifyWikiTableValue(args[len(args)-1])
+		checked := false
+		for _, value := range args[:len(args)-1] {
+			if !presentWikiValue(value) {
+				continue
+			}
+			checked = true
+			hidden, err := p.hiddenWikiReference(value, targetDataset)
+			if err != nil {
+				return nil, err
+			}
+			if !hidden {
+				return false, nil
+			}
+		}
+		return checked, nil
 	default:
 		return nil, fmt.Errorf("unknown helper %q", name)
 	}
 }
 
+func (p *wikiExprParser) hiddenWikiReference(value any, targetDataset string) (bool, error) {
+	targetDataset = strings.TrimSpace(targetDataset)
+	if !isWikiVisibilityDataset(targetDataset) {
+		return false, fmt.Errorf("unknown reference dataset %q", targetDataset)
+	}
+	if !presentWikiValue(value) {
+		return false, nil
+	}
+	if p.ctx == nil {
+		return false, nil
+	}
+	key := resolveReferenceKey(value, p.ctx.wikiIDMapForDataset(targetDataset))
+	if key == "" {
+		return true, nil
+	}
+	if !strings.HasPrefix(key, targetDataset+":") {
+		return true, nil
+	}
+	return !p.ctx.canReferenceWikiKey(key), nil
+}
+
 func presentWikiValue(value any) bool {
 	if isNullLike(value) {
 		return false