ci: add IPC TritRPC execute conformance lane

[mdheller]

Summary

Adds the final execute-mode conformance lane for IPC v0 to TritRPC v1 fixture verification.

What lands

• .github/workflows/tritrpc-exec-conformance.yml
• docs/runbooks/ipc-tritrpc-exec-conformance.md

Why

The IPC bridge now exposes tritrpc.fixture.verify, and the Rust CLI wrapper supports execute mode. This PR adds a CI/runbook path that checks out the canonical TriTRPC repo at a pinned commit, builds the Rust trpc CLI, and invokes the IPC bridge operation with explicit local paths.

Boundary

• no fixture bytes are vendored
• TriTRPC is checked out at pinned commit 58741244057ed1346676c7b95c9a1ec940f12952
• no network transport is implemented
• no auth/session protocol is introduced
• this is codec/fixture conformance only

Receipts

The lane expects:

• reference/ipc-v0/.workstation/reports/ipc-tritrpc-exec/run-receipt.json
• reference/ipc-v0/.workstation/test-reports/tritrpc-rust-cli-check-exec.json

Follow-on

• upload receipts as CI artifacts
• pin GitHub Actions by commit SHA
• add a future tritrpc.frame.pack IPC operation