[SoundMatt]

Summary

Implements ROADMAP.md milestone 53, "Power Management Rebuild (v2.9.0)", opening Phase 14. This is a REPLACE of rcp/powerstate.hpp per the Satellite Package Disposition table: the pre-replacement ad-hoc Active/Sleeping/BusOff model built on rcp.hpp's Zone/CommandType/Controller is discarded entirely, not adapted, following the OPEN Alliance TC18 Remote Control Protocol Specification v0.5.1_RC's actual power-mode behavior.

• PowerMode gains the specification's actual four modes: Normal/StandBy/Sleep/Unpowered.
• start_kind_on_exit fixes StandBy as always a hot start and Sleep as always a cold start (PowerManager::pending_start_kind()).
• PowerManager::enter_standby/enter_sleep apply the three documented entry-refusal conditions in a fixed order: an unacknowledged wake-up event (queried directly from rcp::wakeup::WakeupEndpoint::wakeup_message_pending(), v2.7.0), a non-idle endpoint, and a non-empty response/ack queue — the latter two exposed as caller-supplied Hooks::endpoints_idle/Hooks::response_ack_queues_empty predicates rather than a direct rcp/regmap.hpp dependency, mirroring rcp/lifecycle.hpp's (v2.1.0) PlausibilityCheck pattern.
• resume_from_standby implements StandBy's hot-start rule directly — no handshake, straight back to Normal.
• begin_wake_from_sleep/note_wakeup_attempt_sent/acknowledge_wakeup implement the hot-start-from-Sleep handshake by directly driving WakeupEndpoint::wakeup_message_pending()/acknowledge_wakeup() in a repeat-until-echoed-or-limit loop, with Hooks::reenable_network_interface/Hooks::reenable_response_ack_queues covering the handshake's network-reenablement and queue-reenablement steps.
• notify_power_removed/notify_power_restored model Unpowered as a hardware-driven, unconditional transition with no refusal path.

Nothing else in the tree depended on the old powerstate::Manager API (only its own test did), so no legacy shim was needed, same precedent as rcp/e2e.hpp's REPLACE at v2.6.0.

New coverage lives in tests/test_powerstate.cpp (REQ-PWR-001..014, entirely rewritten), traced in .fusa-reqs.json. Bumps kVersion/project version to 2.9.0. Updates ROADMAP.md with a Done (v2.9.0) summary for milestone 53.

Per the confidentiality constraint on the source specification, no spec text is reproduced anywhere in code/comments/this description — only cited by name.

Test plan

• cmake --build build --parallel — full local build (AppleClang 21, C++17 and C++20) green, no new warnings under -Wall -Wextra -Wpedantic -Wshadow -Wnon-virtual-dtor -Wold-style-cast -Wcast-align -Wunused -Woverloaded-virtual
• ctest --test-dir build --output-on-failure — all 59 test suites pass (including rcp_powerstate, 17 cases / 78 assertions)
• cpp-rcp CLI target still builds (RELAY_BUILD_CLI=ON)
• .fusa-reqs.json is valid JSON; REQ-PWR-001..014 traced 1:1 against tests/test_powerstate.cpp's fusa:test tags
• CI (cross-platform build/test matrix, RELAY conformance gate, clang-tidy, coverage, cpp-FuSa checks) — awaiting green run on this PR