docs: retire the five design memos — implemented work verified, remainders tasked

Audited each design-*.md against the tree (symbols, wiring, receipts), not
against its own status lines — two of them undersold what had landed
(tiered-kv's 'built, unwired' table rows: kv/radix now wired via
kv/prefixindex, kv/kvtier via model/state/ramspill into serve; the CB memo's
kv/budget 'unwired' claim: wired via serving/pipeline + lane_set).

- design-continuous-batching: unified scheduler (3 modes, --scheduler),
  laneSet multi-session owner (2.58x aggregate receipt), weight-read-once
  GEMM incl. quant + q8-KV — ALL LANDED. Remainder (non-ICB arches, slice-2
  prefill admission + batched head, serve rungs, continuity cancel) → task #8.
- design-dflash: contract slice + engine forward landed; live lane gated on
  the non-corrupting aux tap exactly as documented → task #4.
- design-tiered-kv: everything landed except caching the declined request
  classes → task #5.
- design-family-roadmap: 26 family lanes landed (ranks 1-11, 13, 14, 18/19
  one-offs); the live queue remainder → task #6.
- design-quant-formats: research only; the exporter decision + ranked build
  order → task #7.

Full texts remain in git history.

Co-Authored-By: Virgil <virgil@lethean.io>

Author: Snider