faster-boto3: build parity fixture corpus and shadow-mode harness for native vs legacy paths

[justrach]

Goal

Build the compatibility harness that gates the native migration.

Requirements

• Support legacy, native_shadow, and native modes
• Record canonical request/response fixtures per operation
• Compare parsed success outputs, parsed errors, headers, timestamps, and streaming semantics
• Run against LocalStack and real AWS for the migrated operations
• Store mismatch corpus for regression tracking

Working Branch

• native-boto-3

Why This Matters

This is the core migration safety rail. Native coverage should not expand without this harness.

Blocked By

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

[justrach]

Blocked by: #65.