Consolidate docs: merge DESIGN.md into README; sharpen status + roadmap

DESIGN.md was ~80% redundant with README.md (project summary, hardware,
API contract, callback contract, scrypted integration, milestone
acceptance criteria) and ~20% implementation guidance that's now better
served by the source code itself + TESTING.md. Removed it and folded
the unique parts into README.

README.md additions:
- "Status" table at the top: firmware, Scrypted side, hardware — each
  with where-we-are + what's-pending columns.
- "Firmware implementation notes" section: source map (one-line per
  main/*.{h,c} module), memory strategy, display strategy, error
  handling, coding standards. Replaces DESIGN.md sections 7-11, 15, 17.
- "What's next" section before Philosophy: nine items in priority
  order, each linking the specific TESTING.md or scrypted/ section.
  Makes "pick up cold" trivial.
- Philosophy section absorbed DESIGN.md's "Guiding Rule" as a
  blockquote at the bottom.
- "Related docs" rewritten — TESTING.md is now described as *the*
  self-contained verification reference.

TESTING.md additions:
- Preamble now describes the five-layer structure and the
  ⬜/🟡/✅ legend.
- New "Status snapshot" table — milestones + code + HW at a glance.
- New "Hardware prerequisites" — table of what to confirm/order,
  plus a numbered list of the four open unknowns (DSI FPC pin count,
  I²C jumper destinations, BOOT GPIO, flash silkscreen).
- New "Recommended bench order" — seven stages from "board on
  Ethernet over USB" through "integration suite," each with the
  commands or where-to-look.

DESIGN.md deleted. Single source of truth is now README.md (spec +
implementation notes + roadmap) and TESTING.md (verification
reference).

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