proposal: expose in-flight execs via /debug/execs on debugaddr

[ZRHann]

Description

Add a read-only JSON endpoint /debug/execs to the existing debugaddr HTTP
mux (alongside /debug/pprof and /metrics) listing exec ops that currently
hold or are waiting for the parallelism semaphore (--oci-max-parallelism).

Example response:

{
  "limit": 8,
  "running": [
    {"vertex": "sha256:abc...", "name": "[3/10] RUN mamba install ...",
     "traceID": "97b627...", "since": "2026-08-28T06:12:42Z", "age": "9m32s"}
  ],
  "waiting": [
    {"vertex": "sha256:def...", "name": "[2/8] RUN apt-get install ...",
     "traceID": "4af572...", "since": "2026-08-28T06:21:50Z", "wait": "1m05s"}
  ]
}

Would a PR adding this be accepted?