docs: introduce TerseSpec in README (en/zh/ja)

Adds a TerseSpec section to the developer part of the README: what it
is (hand-writable authoring grammar expanded by normalize() into the
canonical chart), a GraphicRenderer usage snippet, a grammar-at-a-glance
table, the unified-entry behavior of normalize(), and the
authoring-only/persistence-unchanged guarantee, with links to the
playground lessons, docs/terse-spec-design.md and the JSON Schema.
Also adds a main-features bullet and points the deprecated spec prop
doc at TerseSpec.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>