refactor(bridge): encapsulate lifecycle registration

[Andiveli]

Summary

• Give QR-channel and event-handler registration lifecycle one synchronized owner.
• Make repeated connection setup register the WhatsApp event handler only once.

Changes

• Add mutex-protected lifecycle state for QR and registration state.
• Reset lifecycle registration when constructing a new client.
• Preserve existing cgo exports, callback order, reconnect behavior, and the parent ownership contracts.
• Add focused idempotency and reset coverage.

Testing

• cargo fmt --check
• cargo test -- --test-threads=1 (604 passed)
• Focused lifecycle tests (2 passed)
• cd whatsrust/lib && go test ./... -count=1
• cd whatsrust/lib && go vet ./...
• git diff --check
• Manual testing: N/A; internal lifecycle extraction.

Chain context

main
└── PR #260: cgo ownership
    └── 📍 refactor/go-client-lifecycle
        └── next: migrate direct client consumers behind accessors

• Depends on: PR refactor(bridge): make cgo ownership explicit #260 / refactor/go-cgo-ownership.
• Ends with: idempotent lifecycle registration and owned QR state.
• Follow-up: migrate selected package-level client consumers without broad event-routing changes.
• Out of scope: panic mapping, identity policy, outbound builders, and general event routing.
• Review budget: 78 authored lines.
• Rollback: revert 9a543c6; only three lifecycle/event-wiring files are affected.

Closes #262