spike: demonstrate RFC-0001 payload-integrity fallback

[binarylogic]

Why

This spike demonstrates one concrete principle from RFC-0001 (Edge Proxy Runtime Contract):

• if transformed payload integrity is uncertain, do not forward risky mutated data
• fallback to original payload behavior instead

RFC reference: https://github.com/usetero/rfcs/pull/1

What this spike changes

When a module returns a modified payload and recompression fails, the proxy now bypasses the modified payload and forwards the original request body unchanged.

Why this is useful

This is a small, isolated example of the runtime contract in action:

• integrity over best-effort mutation
• explicit safe fallback
• no broad Theme 1 refactor

Validation

• zig build test

[github-actions]

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