Add FileSystemHandle.prototype.getUniqueId() per WHATWG FS spec

[Copilot]

Implements FileSystemHandle.prototype.getUniqueId() — a stable, per-entry UUID returned as a UUID v4-format string.

Design

Identity strategy per adapter:

Adapter	Strategy
memory.js	No change — same-entry handles share the same JS object, so the fallback caches correctly via object identity
node.js	SHA-256(kind:path) via node:crypto createHash
deno.js	SHA-256(kind:path) via crypto.subtle.digest
indexeddb.js	IDB auto-increment key hex-formatted as UUID, prefixed with f/d per kind
cache.js	SHA-256(kind:path) via crypto.subtle.digest

FileSystemHandle.js fallback — if the adapter has no getUniqueId(), a random UUID is generated once and cached on the adapter object:

async getUniqueId () {
  const adapter = this[kAdapter]
  if (adapter.getUniqueId) return adapter.getUniqueId()
  if (!adapter._uniqueId) adapter._uniqueId = crypto.randomUUID()
  return adapter._uniqueId
}

Including kind in the hash input is the key correctness detail: it ensures a file and a directory that transiently share the same path produce distinct IDs, satisfying the WPT test "a file and a directory of the same path return different IDs".

Tests

FileSystemBaseHandle-getUniqueId.js moved from SKIP_REASONS → SUPPORTED_SCRIPTS in both wpt-node.js and wpt-browser.spec.js. All 11 WPT subtests pass against both the memory and Node.js adapters.