[SoundMatt]

Summary

Implements ROADMAP.md milestone 46, "Discovery (v2.2.0)": the mandatory
discovery request and discovery-stream claiming mechanism from the OPEN
Alliance TC18 Remote Control Protocol Specification v0.5.1_RC.

• New include/rcp/discovery.hpp:
  • make_discovery_request / encode_discovery_request: an unconditional
    ACF_ABB read targeting byte_bus_id 0 (kDiscoveryByteBusId, matching
    regmap::kEp0) at register-map address 0 (kDiscoveryRegisterAddress),
    always wrapped in an NTSCF header. Rides directly on rcp/wire.hpp's
    existing NTSCF/ACF_ABB framing (v2.0.0) — no changes to that codec.
  • decode_discovery_request: a TSCF-headed discovery request is dropped
    as a decode failure (DiscoveryErrc::tscf_headed_request_dropped), not
    a boolean flag a caller could forget to check.
  • DiscoveryClaim: the first discovery request accepted while
    rcp::lifecycle::ServerLifecycle reports HwUnconfigured/
    HwConfigured reserves the discovery stream for that client's
    configuration writes; the reservation lapses after a configurable
    Discovery_TimeOut (kDefaultTimeout, ~20ms default) if no
    configuration request follows; other clients' concurrent discovery
    requests never dislodge an active claim, nor does an active claim ever
    block their own reads from being answered; a request in RcpConfigured
    is always NotEligible, since claiming is scoped to the two
    pre-RCP-configured states.
  • should_answer_discovery: a single, always-true, testable call site
    documenting the mandatory any-state answering invariant.
• New tests/test_discovery.cpp (REQ-DISC-001..009), registered in
  tests/CMakeLists.txt following the existing test_wire/test_lifecycle/
  test_regmap pattern.
• .fusa-reqs.json: 9 new traced requirements (REQ-DISC-001..009).
• ROADMAP.md: milestone 46 gets a "Done (v2.2.0)" paragraph following the
  milestone 44/45 precedent.
• Version bump: include/rcp/version.hpp and the CMake project version to
  2.2.0.

No existing header needed changes — rcp/wire.hpp, rcp/lifecycle.hpp, and
rcp/regmap.hpp are used as-is, per the roadmap's own dependency note for
this milestone.

Test plan

• cmake --build (system AppleClang, matching CI's clang-14 config)
  builds the full tree with zero warnings on the new files under the
  repo's strict -Wall -Wextra -Wpedantic -Wshadow ... flags.
• ctest full suite: 44/45 tests pass, including the new rcp_discovery
  (16 test cases / 50 assertions). The one non-pass
  (rcp_powerstate) is a pre-existing, unrelated timing/threading test
  that I killed manually after it stalled in this sandbox — unrelated to
  this change (confirmed pre-existing on main before this branch).
• .fusa-reqs.json remains valid JSON with the new REQ-DISC-* entries
  traced to tests/test_discovery.cpp.
• CI (build-and-test matrix, relay-conform, clang-tidy, cpp-FuSa gates)