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/lib/pipeline/index.ts

@@ -6,3 +6,4 @@ export * from "./axis.js";
 export * from "./crop.js";
 export * from "./pricing.js";
 export * from "./thicknessBand.js";
+export * from "../validation/index.js";