docs: mark Phase 5 complete and record build verification results

[jwoglom]

Motivation

• Update the project doc to reflect that the watch-as-host Phase 5 surface is complete and to capture the remaining verification gap.
• Record the outcome of a build-and-unit-test closeout run performed in this environment and preserve the open xDrip+ runtime validation item.
• Capture the decision to defer deep service-internals extractions to a post-merge Phase 6 hardening pass.

Description

• Marked Phase 5 (watch pump-host UI) and all sub-steps 5a–5f as complete in the watch-as-host refactor plan and tightened several phase-status lines and wording.
• Replaced the previous "Build/verification gaps" section with a concrete "Build/verification status" note that records a successful local closeout run and highlights the remaining xDrip+ receiver-side smoke-test requirement.
• Documented the decision to defer PairingManager and WearMessageForwarder extractions to a post-merge Phase 6 maintenance pass.
• Small editorial and formatting cleanups in the plan, plus addition of the audited Tier summary and verification pattern clarifications.

Testing

• Ran ./gradlew :mobile:assembleDebug :wear:assembleDebug :shared:testDebugUnitTest :db:testDebugUnitTest --console=plain and the build plus unit-test closeout suite passed end-to-end in this environment.
• The ADB/on-device xDrip+ receiver smoke test was not executed because adb devices -l returned no connected devices, so that runtime validation remains open.
• No other automated tests were changed or failed as part of this documentation update.

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Codex Task