feat: add routing policy commands

[alanshaw]

Adds:

• /routing/put - add/update a routing policy
• /routing/use - instruct a space to use an existing routing policy

refs https://github.com/fil-one/RFC/blob/main/rfcs/2026-08-routing-policies.md

[Copilot]

🟡 Changes recommended

The new shared DAG-JSON helpers allow unbounded object/array sizes (missing the generators’ 8192-entry limit), creating an avoidable input-parsing DoS risk and inconsistent decoding behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR introduces a new commands/routing package that defines UCAN commands for routing policies (/routing/put and /routing/use), including types, codecs, and generated CBOR/DAG-JSON encoders/decoders. It also refactors existing hand-written codecs to use a new shared internal codec helper package.

Changes:

• Add /routing/put and /routing/use command bindings plus routing policy argument types.
• Add a hand-written codec for DID-keyed CandidateSet, along with generated *_gen.go files and round-trip tests.
• Introduce commands/internal/codec to share CBOR/DAG-JSON helper primitives and update the S3 hand-written codec to use it.

File summaries

File	Description
commands/s3/codec.go	Refactors hand-written codecs to use shared internal codec helpers.
commands/routing/use.go	Adds /routing/use command binding and stable error-name constants.
commands/routing/types.go	Defines routing policy types and command argument structs.
commands/routing/put.go	Adds /routing/put command binding and stable error-name constant.
commands/routing/json_gen.go	Generated DAG-JSON codec for routing types/arguments.
commands/routing/gen/main.go	Code generator entrypoint for routing CBOR/DAG-JSON files.
commands/routing/codec.go	Hand-written codec for DID-keyed CandidateSet.
commands/routing/codec_test.go	Round-trip tests for routing codecs and wire format.
commands/routing/cbor_gen.go	Generated CBOR codec for routing types/arguments.
commands/internal/codec/codec.go	New shared CBOR/DAG-JSON helper primitives for hand-written codecs.

Review details

Files not reviewed (2)

• commands/routing/cbor_gen.go: Generated file
• commands/routing/json_gen.go: Generated file

Suppressed comments (1)

commands/internal/codec/codec.go:194

• ReadJSONArray has no maximum element count, unlike dag-json-gen generated decoders which reject arrays larger than 8192 elements. Adding a limit here avoids unbounded allocation/CPU on malicious inputs and keeps behavior consistent with generator defaults.

	for {
		v, err := read()
		if err != nil {
			return nil, err
		}
		out = append(out, v)
		close, err := jr.ReadArrayCloseOrComma()
		if err != nil {
			return nil, err
		}
		if close {
			return out, nil
		}
	}

• Files reviewed: 8/10 changed files
• Comments generated: 1
• Review effort level: Lite

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