Add typed snapshots for datasource-bound properties

[GG-O-BP]

Summary

• Add mendraw/datasource with typed Snapshot capture for datasource-bound widget properties, distinguishing PropertyAbsent, Loading, Unavailable, and Available with ordered items and stable page metadata.
• Add typed object-array property access (object_list, ObjectListEntry, entry_field_string) for nested configuration such as columns, plus attribute_value to resolve an item-bound attribute as an EditableValue.
• Cover available/partial/empty/loading/absent/unsupported states, object-list errors, and attribute resolution with synthetic fixtures, and document the datasource-bound widget flow in README and guides.

Fixes #25

Test plan

• gleam format --check
• gleam check
• gleam build --warnings-as-errors
• gleam docs build
• gleam test --runtime bun
• ./scripts/verify.sh inner mendraw
• ./scripts/verify.sh shared mendraw — check-all, standalone consumers, all package unit tests, Glendam browser tests (61), and widget build pass; the single-command rerun currently trips a Hex API rate limit in test-standalone-consumers.py (environment issue, each component passed separately)