OpenMetrics endpoint: versioned status/metrics schema (detect shape drift on older hosts)

**Constraint fold-in from Chris Nelson's monitoring feedback (2026-07-15), promoted from the #1636 design comments into a tracked sub-issue so it can't be skipped when the endpoint is built.**

## Problem
~36 older Fabric hosts are currently **blind to the monitoring sidecar because of UDS / status-shape drift** — the status payload shape changed under them with no negotiated version, so collectors silently went dark instead of detecting the skew.

## Ask
The `/metrics` payload (and the underlying status shape it draws from) must carry an **explicit schema version** so scrapers can detect and *tolerate* skew rather than silently failing. Treat cross-version shape compatibility as a first-class design constraint of the endpoint, not an afterthought.

## Acceptance criteria
- The metrics/status payload advertises a schema version a collector can read.
- A collector on an older cohort can detect version skew and flag it (surface a "stale shape" signal) instead of going silently dark.

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🤖 Filed by KrAIs on behalf of Kris.

Start by tracing the /metrics endpoint and the underlying status shape it draws from, then review the #1636 design comments for the compatibility context. Done means the payload exposes a readable schema version and older collectors can detect skew and surface a stale-shape signal instead of failing silently.

Written by the indexing model from the issue text.