feat: add WP.Post / WP.Term / WP.Tax + extend WP.User with REST CRUD

[gin0115]

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).