Add type and filter helpers on Entries

[Flerpharos]

Summary
Add filter helpers on Entries so common queries do not require hand-rolled iteration.

Scope

• add a typed filtering path such as of_type(...)
• add common convenience helpers such as attachments() and texts() if they fit the API cleanly
• document the intended ergonomic read path

Acceptance criteria

• callers can filter entries by type without writing manual loops
• common convenience helpers are covered by docs and tests
• the API remains consistent with the existing collection model