A liveness response joins the /v1 contract

[enesismail]

The contract gains HealthResponse, the body of an unauthenticated liveness
endpoint the control plane will mount. Nothing in this client calls that
route, and the type belongs here anyway: this package defines the protocol
once, before anything serves it, and the callers that need a liveness answer
— a process supervisor, or a person establishing that a listening port is
attached to a server that answers — are as much consumers as the CLI is.

Three fields, every one emitted at its zero value because this package bans
omitempty outright:

• uptime_seconds — a count, not an instant. At zero a count reads as zero,
  where an unset timestamp renders as a date in year one that every reader
  has to decode before dismissing, and a committed fixture is read far more
  often than it is written.
• artefact_sha256 — what the running binary computes of ITSELF at startup.
  Its own identity rather than an assertion about it, which is the one form
  of that fact which cannot disagree with reality. Empty when the process
  could not read its own file, and that is not an error: a liveness endpoint
  which failed on self-inspection would not be one.
• deployed_commit — configuration, and the field name says so rather than
  implying the binary knows. Whatever installed the binary is the only thing
  that knows which revision produced it, so the value is exactly as
  trustworthy as that installation. Deliberately not compiled in: stamping a
  revision would make the binary's bytes depend on where it came from, and
  the build that produces it is pinned precisely so its output follows from
  its inputs alone.

TWO FIXTURES, because the all-zero state is reachable rather than
degenerate. A supervisor's first poll arrives before a second of uptime has
accrued; a binary that cannot read its own file reports no digest; nothing
obliges an installation to declare a revision. All three answer success, so
all three need pinning — the same argument the closed-capacity fixture
already makes, and that fixture's comment claiming to be the only one of its
kind moves in this change rather than being left beside its own
counterexample.

Fixture values are synthetic on purpose. A fixture pins field names; it does
not record a measurement. This repository is world-readable, and a real
digest beside a real revision id would publish two facts about a private
deployment in a file that needed three keys.

Both new golden rows were proved by mutation before being trusted: dropping
a key from the zero fixture reds with "emitted but absent from the fixture",
and tagging a field omitempty reds both new rows with "vanishes when the
zero value is marshalled". Restored byte-identical, with the baseline green
again afterwards, so the restore is measured rather than assumed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XfBYBGuUkF7zUnZfAFD3dW