Implement portable outbound HTTP and request lifecycle contracts #436

Scope

Track the portable HTTP and request-lifecycle foundation implemented by #275 across core, Axum, Cloudflare, Fastly and Spin.

• Buffered and streamed outbound requests/responses, bounded completion-driven batches, cancellation and explicit terminal failures.
• Route-aware ingress admission, bounded reads, application-owned clocks/deadlines and fallible application assembly.
• Adapter-owned response egress, exactly-once completion/resource release, framing validation and safe diagnostic provenance.
• Bounded configuration reads, provider capability declarations and migration of generated applications, fixtures and documentation.

Acceptance

• Review and merge feat: implement portable outbound HTTP and request lifecycle contracts #275 with its documented breaking API migrations.
• Pass the relevant workspace, adapter, generated-consumer and documentation checks against the revision being merged.
• Preserve documented Unsupported/BestEffort distinctions and clearly identify deferred hosted qualification. Local tests and guest telemetry do not prove deployed guarantees or end-client receipt.

The detailed implementation, evidence and remaining qualification boundaries are recorded in #275. The Axum production-readiness work in #391 depends on this foundation and has separate acceptance criteria.