feat(ios): route push notification taps to messages

[square-tomb]

Summary

Adds the missing iOS notification-response path on top of the hardware-proven development push foundation in #2744.

• the Notification Service Extension attaches a verified, local navigation target only after it retrieves and verifies the real relay event
• the Runner buffers cold-start notification responses and bridges warm responses to Flutter
• Flutter parks the response in the existing deep-link pipeline, switches to the originating local community when needed, and opens the exact channel/message
• untrusted navigation metadata from the original APNs payload is stripped before resolution

This is intentionally a draft stacked on push-ios-blockers-1-2 / #2744 so the tap-routing delta stays reviewable. It does not modify or close #2744 or #3347 and does not change rollout state.

Hardware context

The underlying development path is already proven on the connected iPhone 12 mini:

• development relay → development push gateway → sandbox APNs
• NSE fetched/decrypted the real event and visibly rendered its real title/body on the lock screen
• proof event: c678ddbe1a477c3f61056f0fbf13693f00c32873378ea39fa961a58f7c967e79

Physical revalidation of this new tap-routing build remains pending because Xcode's current tomb@squareup.com account token is missing and the Block team shows no Certificates, Identifiers & Profiles access. Automatic provisioning was retried for both the app/NSE and the XCUITest host/runner.

Checks

• just mobile-check
• focused Flutter push/deep-link tests: 16 passed
• BuzzPushKit: 49 XCTest + 3 Swift Testing tests passed
• Xcode 26.6 unsigned iOS Simulator workspace build succeeded
• physical app automatic-signing retry: blocked before compilation by missing team account/profile
• physical XCUITest automatic-signing retry: blocked before test build by missing Xcode token/profile

Scope

Development/internal evaluation only. No production infrastructure, external rollout, feature-flag, or release-state change.