refactor(js): delegate compatible promise ops to gleam_javascript (#12)

[GG-O-BP]

Summary

Closes #12. Delegates the compatible glendix/js/promise operations to
gleam/javascript/promise and removes their duplicate handwritten JavaScript,
keeping only the two adapters the package cannot express.

Changes

• src/glendix/js/promise.gleam — resolve, then_, map, all, race,
  and await_ now delegate to gleam/javascript/promise (resolve, await,
  map, await_list, race_list, and map respectively). reject and
  catch_ remain thin facades over retained FFI adapters, each documented
  in-source. Stale externals removed.
• src/glendix/js/promise_ffi.mjs — trimmed to only promise_reject
  (Error-wrapping rejection construction) and promise_catch (Promise-returning
  recovery); dropped the now-unused toList import and six duplicate functions.
• test/glendix_test.gleam / test/glendix_test_ffi.mjs — added async
  tests for resolve, mapping, Promise-returning chaining, ordered all, race
  (first-fulfilled and first-rejection), Error-surfacing rejection,
  Promise-returning recovery, fulfilled pass-through, and one-shot await_.

Design decisions

• reject: gleam_javascript has no rejection constructor and the reason must
  surface as a JavaScript Error, so a minimal adapter stays.
• catch_: promise.rescue recovers with a plain value; it cannot preserve
  this helper's Promise-returning recovery, opaque PromiseRejection, or native
  catch flattening, so a minimal adapter stays.
• Public API (names, labels, types) is unchanged — non-breaking internal
  refactor, so no migration docs or semver bump are required.
• glendix -> mendraw keeps its declared Hex source form.

Acceptance criteria

• Every helper is mapped to a package primitive or documents a retained adapter
• Compatible operations no longer have duplicate handwritten JS
• Tests cover resolve, mapping, chaining, ordered all, race, rejection, Promise-returning recovery, and one-shot await_
• Rejected string messages still surface as JavaScript Error values
• No public API change (no migration/semver treatment needed)
• FFI contract checks report no stale external declarations or exports
• glendix -> mendraw retains its declared dependency source form

Verification

Run against the isolated worktree/commit:

• gleam format --check, gleam check, gleam build --warnings-as-errors, gleam docs build — pass
• gleam test --runtime bun — 37 passed, no failures
• check-style.py, check-ffi-contracts.py — pass
• dependency-mode.sh check-commit glendix — valid

Note: the shared/final family E2E tier operates on the primary working trees
(currently an unrelated feature branch) and cross-repo browser/Mendix
harnesses; it was not run from this isolated worktree, which has no downstream
consumers of this facade.