[Rebased] feat(heller): add transport admission manifest v0.1

[mdheller]

This is a clean replacement for #51, rebuilt directly on top of current main.

Summary

Adds the first Heller transport admission slice on top of the already-merged Heller transport seed.

Included

• docs/heller-v1-transport-admission-v0.1.md
• fixtures/descriptors/heller/v1/transport_admission_manifest_v0.1.json
• tools/verify_heller_transport_admission.py
• .github/workflows/validate-heller-transport-admission.yml

Why this replacement exists

PR #51 was based on an older main head and is currently reported as non-mergeable. This branch carries the same intended four-file admission slice on top of the newer mainline head used by other current PRs.

What this does

This PR binds the two existing Heller payload families to explicit TriTRPC transport service/method surfaces:

• heller.v1.Transport / EventEnvelope.Avro
• heller.v1.Transport / EventEnvelope.Protobuf
• heller.v1.Transport / StateSnapshot.Avro
• heller.v1.Transport / StateSnapshot.Protobuf

It also records payload codec labels, message refs, sample payload filenames, and sample payload SHA-256 values from encoded_payload_manifest_v8.json.

What this does not do

This PR does not emit canonical TritRPC frame vectors yet.

The next step after this lands is the frame-vector slice currently staged in #59, which should be rebased or recreated on top of this clean branch.

Validation

The new workflow runs:

python tools/verify_heller_transport_admission.py