Gate B #26: shared validation module (V-1, V-2, V-3)

- V-1: SKU charset [A-Z0-9-]{3,24}, accepted/rejected with specific error codes
- V-2: title/short_title/description UTF-8 within WooCommerce maxLength,
  enforced at field-exit and submit (200/200/2500 chars, Unicode code-point counting)
- V-3: validation module version hash, client caches, submit carries hash,
  mismatch returns 412 with fresh module (not 409)

25 unit tests green, zero browser/DOM/network dependencies.
Re-exports through pipeline/index.ts for shared use.

frontend/src/__tests__/fixtures/validation.ts

@@ -0,0 +1,113 @@
+/** Boundary strings for shared validation unit tests (V-1, V-2, V-3). */
+
+// ---------------------------------------------------------------------------
+// V-1: SKU [A-Z0-9-]{3,24}
+// ---------------------------------------------------------------------------
+
+/** Valid SKUs spanning the charset and length boundaries. */
+export const validSkuStrings = [
+  // Minimum length (3 chars) — all letter, all digit, mixed, hyphen edges.
+  "ABC",
+  "012",
+  "A0A",
+  "A-B",
+  "0-1",
+  // Mid-range typical.
+  "BW-0042",
+  "SLAB-UAT-001",
+  "A9Z",
+  // Maximum length (24 chars).
+  "AAAAAAAAAAAAAAAAAAAAAAAA",
+  "A1B2C3D4E5F6G7H8I9J0K-12",
+  // Edge cases within charset.
+  "A-A-A-A-A-A",
+  "1-2-3-4-5-6",
+  "ABCDEF",
+];
+
+/** Invalid SKUs: wrong charset, too short, too long. */
+export const invalidSkuStrings = [
+  // Too short (1–2 chars).
+  "A",
+  "AB",
+  // Too long (25+ chars).
+  "AAAAAAAAAAAAAAAAAAAAAAAAA",
+  // Lowercase letters (outside charset).
+  "abc",
+  "bw-0042",
+  "aB3",
+  // Special characters outside charset.
+  "ABC!",
+  "A B",
+  "A_B",
+  "ABC.D",
+  "ABC#1",
+  "ABC/1",
+  "ABC\\1",
+  // Empty.
+  "",
+];
+
+// ---------------------------------------------------------------------------
+// V-2: Title / short_title / description maxLength (UTF-8)
+// ---------------------------------------------------------------------------
+
+import {
+  MAX_TITLE_LENGTH,
+  MAX_SHORT_TITLE_LENGTH,
+  MAX_DESCRIPTION_LENGTH,
+} from "../../lib/validation/constants.js";
+
+const TITLE_LEN = MAX_TITLE_LENGTH;
+const SHORT_TITLE_LEN = MAX_SHORT_TITLE_LENGTH;
+const DESC_LEN = MAX_DESCRIPTION_LENGTH;
+
+function repeat(char: string, count: number): string {
+  return char.repeat(count);
+}
+
+function buildTitle(): { title: string; length: number } {
+  return { title: repeat("A", TITLE_LEN), length: TITLE_LEN };
+}
+
+function buildShortTitle(): { shortTitle: string; length: number } {
+  return { shortTitle: repeat("B", SHORT_TITLE_LEN), length: SHORT_TITLE_LEN };
+}
+
+function buildDescription(): { description: string; length: number } {
+  return { description: repeat("C", DESC_LEN), length: DESC_LEN };
+}
+
+export function validTitleFixture(): { title: string; length: number } {
+  return buildTitle();
+}
+
+export function invalidTitleFixture(): { title: string; length: number } {
+  return { title: repeat("A", TITLE_LEN + 1), length: TITLE_LEN + 1 };
+}
+
+export function validShortTitleFixture(): { shortTitle: string; length: number } {
+  return buildShortTitle();
+}
+
+export function invalidShortTitleFixture(): { shortTitle: string; length: number } {
+  return { shortTitle: repeat("B", SHORT_TITLE_LEN + 1), length: SHORT_TITLE_LEN + 1 };
+}
+
+export function validDescriptionFixture(): { description: string; length: number } {
+  return buildDescription();
+}
+
+export function invalidDescriptionFixture(): { description: string; length: number } {
+  return { description: repeat("C", DESC_LEN + 1), length: DESC_LEN + 1 };
+}
+
+// ---------------------------------------------------------------------------
+// V-3: Validation module version hash
+// ---------------------------------------------------------------------------
+
+/** Expected hash for the current module (constants v1). */
+export const expectedHash = "v1";
+
+/** A stale hash to test mismatch detection. */
+export const staleHash = "v0";