Adds typed read/write helpers for the standard WP REST resources, all
sharing the WP.Http nonce auth and a consistent module shape.

New modules:

- WP.Post — list/get/create/update/delete against /wp/v2/posts. Handles
  the dynamic shape of the WP REST post object via a `DecodeKeys` record
  the caller passes in:
    * `taxonomies : List String` — top-level taxonomy keys to extract
      (e.g. ["categories", "tags", "genre"]) into a `Dict String (List Int)`.
      Supports any registered taxonomy on any post type, not just the
      built-in `categories` / `tags`.
    * `meta : List String` — post meta keys registered with show_in_rest
      to extract from the response's `meta` object into `Dict String Value`.
  ListParams supports per-page, page, search, status, ordering, author, plus
  per-taxonomy filters (e.g. `?categories=3&genre=5`). Body for create/update
  has the same dynamic shape — set `taxonomies` and `meta` dicts to assign
  on save.
  Includes `taxonomyOf` and `metaOf` accessor helpers.

- WP.Term — list/get/create/update/delete for taxonomy terms. Operates on
  any taxonomy via its rest_base ("categories", "tags", "genre", etc.).
  Supports term meta via the same DecodeKeys + meta pattern.

- WP.Tax — read-only access to /wp/v2/taxonomies. Decodes the keyed-by-slug
  response into a flat `List Taxonomy`.

- WP.User extended — adds restDecoder + DecodeKeys + Body + ListParams and
  list/get/create/update/delete against /wp/v2/users. Maps the REST-side
  "name" field to displayName, fills missing roles/capabilities with empty
  lists for default context, extracts truthy capability keys for edit
  context, and supports user meta via the same DecodeKeys pattern.
  The flags-side `currentUser` decoder gains `optional "meta" ... Dict.empty`
  so the User type stays unified across both sources (PHP-side
  Standard_Flags doesn't currently emit user meta, but the structure is in
  place if we add it later).

Internal modules (not exposed):
- WP.Query — shared URL query string builder with percent-encoding.
- WP.User.Type — defines the User record + flags-side decoder, used by
  both WP.User and WP.Flags to break the import cycle that would
  otherwise exist (User → Http → Flags → User).

Dependencies:
- elm/url added for percent-encoding query params.

Tests:
- 36 new tests across QueryTest, PostTest, TermTest, TaxTest, UserTest.
- Total suite: 52 tests, all passing.

Sandbox elm.json adjusted to add elm/url as a direct dep (the sandbox
reads elm-wp's source directly via source-directories so it inherits the
new dep).