refactor(js)!: replace glendix/js/array with gleam_javascript array (#11)

[GG-O-BP]

Summary

Closes #11. Delegates glendix/js/array conversions to gleam/javascript/array
and removes the handwritten JavaScript adapter, so Glendix no longer owns
duplicate array runtime code.

Changes

• src/glendix/js/array.gleam — from_list and to_list now delegate to
  gleam/javascript/array.from_list/to_list. Their names, labels, and
  element-order behavior are unchanged, so list |> array.from_list |> array.to_list
  keeps working. The former opaque JsArray(element) type is removed and its
  two private @external declarations are gone.
• src/glendix/js/array_ffi.mjs — deleted; nothing references it.
• test/glendix_test.gleam — round-trip tests now cover empty, singleton,
  and ordered Int lists plus a String list for generic-element typing.
• gleam.toml + READMEs/guides — version bumped to 6.0.0 and install
  ranges updated; a migration note documents the JsArray removal.

Why JsArray was removed (not kept as a facade)

The issue prefers preserving JsArray as a compatibility facade "if Gleam's
type-alias/export rules permit it." A transparent re-export would require a
pub type JsArray(element) = gleam/javascript/array.Array(element) alias, but
the family's mechanical style gate (check-style.py, run by
./scripts/verify.sh shared) forbids all type X = aliases. Since this is a
public/shared boundary change, that gate must pass, so the alias cannot merge.

Per the issue's fallback ("document the required migration and treat the change
according to semantic-versioning policy"), the opaque type is removed and this
is treated as a breaking change:

• Normal usage through from_list/to_list is source-compatible (the type was
  opaque, so callers never constructed or matched it).
• Only callers that explicitly annotated array.JsArray(T) must switch to
  gleam/javascript/array.Array(T).
• Package bumped 5.1.0 -> 6.0.0; migration documented in README*.md.

Acceptance criteria

• from_list/to_list delegate to gleam/javascript/array; no handwritten JS remains
• src/glendix/js/array_ffi.mjs removed
• Unavoidable source incompatibility (removal of opaque JsArray) documented and versioned
• Round-trip tests cover empty, singleton, and ordered multi-element arrays (plus a second element type)
• FFI contract checks report no stale external declarations or exports
• glendix -> mendraw retains its declared Hex dependency source form

Verification (isolated worktree pinned to the pushed commit)

• ./scripts/verify.sh inner glendix — pass (52 tests, --runtime bun)
• scripts/check-style.py, scripts/check-ffi-contracts.py, scripts/check-boundaries.py — pass
• scripts/family.sh deps-status — Summary: valid (mendraw stays hex)
• dependency-mode.sh check-commit glendix <tip> — valid

Note: ./scripts/verify.sh final (Lustre + full Mendix runtime E2E) was not
run — the managed Mendix toolchain (mx/mxbuild) is unavailable in this
environment. This change is a pure JavaScript-target FFI refactor with no
Mendix/Lustre runtime behavior change, so no Mendix/Lustre compatibility is
claimed beyond the source gates above.