Document ui_multi_select params + assert reference/helper parity

[tylercschneider]

Closes #140

What

The shipped reference (lib/keystone_ui/reference/guide.md, synced into consuming apps via the_local) documented ui_multi_select as a bare (**args) stub, hiding its real signature. This flesh­es it out and adds a guard so the reference can't silently drift from the helper module again.

Findings vs. the issue

The issue listed four omissions (ui_column_picker, ui_multi_select, ui_progress, ui_radio_card) against a stale path (lib/keystone_ui/the_local/agents/keystone-usage.md). On the current reference (reference/guide.md), three of the four were already documented — only ui_multi_select remained a non-informative (**args) stub. That one gap is fixed here; the durable win is the new parity test.

Changes

• reference/guide.md — ui_multi_select(**args) → ui_multi_select(name:, label:, options:, selected: []) with a params note (mirrors the component's real initializer).
• Rebuilt committed locals (the_local/agents/*.md) via rake the_local:build so the synced reference matches.
• reference_test.rb — new test_reference_documents_every_ui_helper enumerates every ui_* method on KeystoneUiHelper at runtime and asserts each appears in the reference, so future helpers can't drift out of the docs undetected. Plus a targeted assertion on the ui_multi_select signature.

Tests

bundle exec rake test → 430 runs, 0 failures. Parity test verified red (naming the omitted helper) when a mention is removed, green when present.