fix: leave omitted codec limits unbounded

[puffball1567]

Summary

• make the omitted CodecLimits policy unbounded, subject to platform resource limits and int width
• retain strict enforcement for every explicitly supplied limit
• add unlimitedCodecLimits and regression coverage
• document that network-facing integrations own and must pass their resource policy
• clarify the equivalent NULL C ABI limits behavior

Verification

• nimble verify -y

This is a v0.3.1 policy correction: local converters retain unrestricted behavior, while Joubako and other network-facing integrations select explicit limits.