docs: rewrite DESIGN.md around concepts and rationale

Restructure from first principles: document the mental model and design
rationale, not struct field names. The running example (R1/R2/R3) is
preserved but uses conceptual language that survives refactors.

New "Why It's Fast" section replaces scattered Deep Dives with
problem→solution format: density-based dispatch, generation-based state,
direct-rule bypass, fused transform-scan, no-op folding, split-borrow.

Cut implementation-level details that duplicate code comments (exact bit
layouts, byte sizes, field names). 388→283 lines.

Author: Foster Guo