Add Space::active, Document::{active, pages_in}, Revision::parents and
RevisionIndex::{active, resolve_active}, replacing the hand-rolled default-
context lookups, page filters and parent walks across the core, offline,
notebook and SMB crates. Derive Clone and PartialEq for Layout, Format and
Tag and read table-cell indents (0x1c001c12), matching the canvas branch.

Validation: 298 workspace tests, Clippy, fmt and the fuzz workspace build.

Assisted-by: claude-fable-5.1