Add fmrest privilege troubleshooting doc with three screenshots

[deffenda]

Actionable error messages from FM-code mapping need a stable destination URL with visual instructions. FileMaker's privilege UI is buried three menus deep. Single markdown doc with three captioned screenshots (Manage Security > Privilege Set > Extended Privileges checkbox) gives confused first-time user a one-page path to a working connection.

{
  "scope": [
    "Create extension/docs/FMREST_PRIVILEGE.md with three sections matching three screenshots: (1) Open Manage > Security",
    "(2) Edit user's Privilege Set",
    "(3) Tick 'Access via FileMaker Data API (fmrest)' under Extended Privileges. Each section under explicit #error-212 / #error-9 anchor. Capture three PNG screenshots from FileMaker Pro session",
    "commit under extension/media/fmrest-privilege/ (01-manage-security.png",
    "02-privilege-set.png",
    "03-extended-privileges.png)",
    "each <=200KB. Update Troubleshooting table in extension/docs/USER_GUIDE.md to add 'FM error 212 / fmrest privilege' row + replace vague 'HTTP 401' row with one distinguishing bad-password from missing-fmrest. Wire docsUrl in normalizeError.ts to point at extension/docs/FMREST_PRIVILEGE.md#error-212."
  ],
  "depends_on": [],
  "done_when": [
    "extension/docs/FMREST_PRIVILEGE.md exists with three embedded screenshots that render in GitHub preview, #error-212 + #error-9 anchors reachable, USER_GUIDE.md Troubleshooting links new doc, clicking docsUrl produced by normalizeError for code-212 opens relevant anchor."
  ],
  "validation": {
    "local": [],
    "review": []
  },
  "references": [],
  "surfaces": [],
  "artifact_expectations": [],
  "review_points": []
}