faster-boto3: rollout controls and feature-parity docs for native backend migration

[justrach]

Goal

Turn the native migration into an explicit compatibility program rather than an implicit refactor.

Scope

• Env var and config gates for legacy, native_shadow, and native
• Per-service and per-operation allowlists
• Mismatch logging and telemetry hooks for shadow execution
• Feature matrix and unsupported-feature list in repo docs
• Promotion gates before an operation can move from shadow to native

Working Branch

• native-boto-3

Reference

• faster-boto3/NATIVE_MIGRATION.md

Blocked By

• [Epic] faster-boto3: native S3 execution path that bypasses botocore request execution #65

[justrach]

Blocked by: #65.