[SoundMatt]

Summary

A conformance gap analysis against the real OPEN Alliance TC18 Remote
Control Protocol Specification v0.5.1_RC found that this repo's existing
RCP implementation shares nothing at the wire level with the real
standard it's named for — it's a self-consistent but entirely home-grown
Zone-addressed Command/Response/Status protocol over a bespoke
16-byte header. This PR replaces ROADMAP.md with a phased plan for a
full, breaking replacement of the core protocol so cpp-RCP's RCP
eventually is the OPEN Alliance TC18 protocol, not a look-alike.

Docs-only change — no code in this PR.

• Sequences the core-protocol replacement in dependency order (Phase 13,
  v2.0.0–v2.8.0): IEEE 1722 wire framing → RC Server lifecycle/register-map
  → discovery → basic endpoints (GPIO/SPI, then I²C/UART/ADC/PWM) →
  conditional requests & sequencers → E2E CRC safe points & safety
  requests → remaining endpoints (LIN/CAN incl. CAN XL/ISELED/MDIO/Wakeup,
  DAC explicitly out of scope) → an explicit fragmentation go/no-go call
  (no-go for this cycle, justified)
• Gives every satellite package (~40 headers under include/rcp/ and
  include/relay/) an individual REPLACE / ADAPT / DEPRECATE / KEEP AS-IS
  call with a one-line reason, plus dependency-ordered migration phases
  (14–16) for the safety-critical rebuilds, transport/bridge rebinds, and
  a deprecation + certification-refresh sweep, ending at a v3.0.0 GA tag
• States plainly, up front, that this breaks every current consumer of
  Zone/Command/Response/Controller/Registry, and explains why no
  compatibility shim is planned
• Preserves the pre-replacement roadmap as an appendix for historical
  record

Test plan

• Docs-only change; no build/test impact expected
• CI (build/test matrix, RELAY conformance gate, clang-tidy, coverage) green
• DCO check green