Implement Cloudflare Email Sending and event adapters

[yhay81]

Outcome

Add Cloudflare Email Sending as the second provider proof while declaring documented capability differences honestly.

Scope

• Email Sending binding transport
• privacy-safe documented error normalization
• provider message ID recording
• per-domain event subscription consumption
• event ID, message ID, and recipient correlation to the canonical ledger
• capability and conformance report

Acceptance criteria

• delivered, deferred, bounced, failed, rejected, and complained events normalize correctly
• unsupported open/click and provider idempotency are reported as unsupported
• duplicate/out-of-order Queue deliveries converge
• 50-recipient and 5 MiB boundaries fail before durable commit
• official Resend SDK requests work unchanged
• Cloudflare remains labeled Beta until evidence supports otherwise

Depends on D1, R2, and Queues adapters.
Parent direction: #81
Execution plan: #84

[yhay81]

Post-merge AWS regression evidence

The Cloudflare Email Sending/event adapter merge was revalidated against the dedicated empty hayainc-test AWS account on the exact merged main commit.

• merged main: 9b534ba7e0900187449643fba1c27ae034df651a
• AWS integration: https://github.com/haya-inc/hayasend/actions/runs/30278763262
• result: success
• dedicated account gate: passed (HayaSendGitHubIntegration, ap-northeast-1)
• API exercise: passed
• durable outbox recovery: passed
• retain_stack=false cleanup: passed
• ephemeral stack: hayasend-it-30278763262-1
• residue checks: stack absent; legacy and managed log-group prefixes empty; payload objects/bucket and data table deleted

This is regression evidence for #103 only. It does not substitute for the separate disposable Cloudflare account proof required by #104.