diff --git a/src/common/utils/accessibility-preferences.ts b/src/common/utils/accessibility-preferences.ts
new file mode 100644
index 00000000..a7eaaea2
--- /dev/null
+++ b/src/common/utils/accessibility-preferences.ts
@@ -0,0 +1,30 @@
+/**
+ * Validate and apply player accessibility preferences. (#419)
+ */
+export interface AccessibilityPreferences {
+  highContrast: boolean;
+  textToSpeech: boolean;
+  fontSizeScale: number;
+  colorBlindMode: "none" | "protanopia" | "deuteranopia" | "tritanopia";
+}
+
+export const DEFAULT_ACCESSIBILITY_PREFERENCES: AccessibilityPreferences = {
+  highContrast: false,
+  textToSpeech: false,
+  fontSizeScale: 1,
+  colorBlindMode: "none",
+};
+
+export function validateFontSizeScale(scale: number): boolean {
+  return scale >= 0.75 && scale <= 2.0;
+}
+
+export function mergeAccessibilityPreferences(
+  overrides: Partial<AccessibilityPreferences>,
+): AccessibilityPreferences {
+  const merged = { ...DEFAULT_ACCESSIBILITY_PREFERENCES, ...overrides };
+  if (!validateFontSizeScale(merged.fontSizeScale)) {
+    merged.fontSizeScale = DEFAULT_ACCESSIBILITY_PREFERENCES.fontSizeScale;
+  }
+  return merged;
+}
diff --git a/src/common/utils/bandwidth-adaptive-payload.ts b/src/common/utils/bandwidth-adaptive-payload.ts
new file mode 100644
index 00000000..9003ff7d
--- /dev/null
+++ b/src/common/utils/bandwidth-adaptive-payload.ts
@@ -0,0 +1,21 @@
+/**
+ * Select a mobile-appropriate response payload variant based on the
+ * client's reported network condition. (#420)
+ */
+export type NetworkType = "wifi" | "4g" | "3g" | "2g" | "offline";
+export type PayloadVariant = "full" | "reduced" | "minimal";
+
+export function selectPayloadVariant(network: NetworkType): PayloadVariant {
+  if (network === "wifi" || network === "4g") return "full";
+  if (network === "3g") return "reduced";
+  return "minimal";
+}
+
+export function shouldDeferSync(network: NetworkType): boolean {
+  return network === "offline" || network === "2g";
+}
+
+export function estimatedPayloadSizeKb(variant: PayloadVariant, baseSizeKb: number): number {
+  const factor = variant === "full" ? 1 : variant === "reduced" ? 0.5 : 0.2;
+  return Math.round(baseSizeKb * factor);
+}
diff --git a/src/common/utils/search-relevance-scorer.ts b/src/common/utils/search-relevance-scorer.ts
new file mode 100644
index 00000000..e457db99
--- /dev/null
+++ b/src/common/utils/search-relevance-scorer.ts
@@ -0,0 +1,34 @@
+/**
+ * Score search result relevance using term-frequency overlap, for the
+ * advanced search and discovery feature. (#421)
+ */
+export interface SearchableDocument {
+  id: string;
+  title: string;
+  tags: string[];
+}
+
+function tokenize(text: string): string[] {
+  return text.toLowerCase().split(/\W+/).filter(Boolean);
+}
+
+export function scoreSearchResult(query: string, doc: SearchableDocument): number {
+  const queryTerms = tokenize(query);
+  const titleTerms = tokenize(doc.title);
+  const tagTerms = doc.tags.map((tag) => tag.toLowerCase());
+
+  let score = 0;
+  for (const term of queryTerms) {
+    if (titleTerms.includes(term)) score += 2;
+    if (tagTerms.includes(term)) score += 1;
+  }
+  return score;
+}
+
+export function searchAndRank(query: string, documents: SearchableDocument[]): SearchableDocument[] {
+  return documents
+    .map((doc) => ({ doc, score: scoreSearchResult(query, doc) }))
+    .filter((entry) => entry.score > 0)
+    .sort((a, b) => b.score - a.score)
+    .map((entry) => entry.doc);
+}
diff --git a/src/common/utils/test-data-factory.ts b/src/common/utils/test-data-factory.ts
new file mode 100644
index 00000000..4f9db127
--- /dev/null
+++ b/src/common/utils/test-data-factory.ts
@@ -0,0 +1,22 @@
+/**
+ * Generic test data factory to reduce fixture boilerplate. (#416)
+ */
+export type FactoryDefaults<T> = () => T;
+export type FactoryOverrides<T> = Partial<T> | ((base: T) => Partial<T>);
+
+export function createFactory<T>(defaults: FactoryDefaults<T>) {
+  return function build(overrides: FactoryOverrides<T> = {}): T {
+    const base = defaults();
+    const patch = typeof overrides === "function" ? overrides(base) : overrides;
+    return { ...base, ...patch };
+  };
+}
+
+export function buildMany<T>(build: (overrides?: FactoryOverrides<T>) => T, count: number): T[] {
+  return Array.from({ length: count }, () => build());
+}
+
+export function sequence(prefix: string): () => string {
+  let counter = 0;
+  return () => `${prefix}-${++counter}`;
+}