atelet: resolve an actor's sandbox runtime from the request's sandbox_runtime - #2788

…t in the metrics suite (agent-substrate#2792)

> agent-substrate#2756 has merged; this is one commit on `main`.

The page describes agent-substrate#2756's final design: the default runtime is linked
under the worker's own directory, in a directory per default the
worker's file names, made and removed by the syncer and gone with the
pod, so the on-disk table, syncer section, eviction rule, and runbook
follow that layout and a subsection gives its lifecycle; and with no
budget set atelet starts no eviction pass.

Step 6 of agent-substrate#2675, in its small form: the e2e under eviction pressure was
dropped as too much machinery for what it proves, and this is what
remained worth doing.

**Metrics suite.** `ate_sandboxassetcache_size` joins
`PlatformMetricPrefixes`, so `TestPlatformMetricsEmitted` proves on
every kind lane that atelet registers the instrument, its callback walks
a real cache directory without error, and the OTLP path to the collector
carries it. Only size qualifies: it is an observable instrument read on
every collection, so it has a series from the first export on any
install. The eviction and pass counters emit only once a pass had work
to do, which never happens with the default unbounded budget.

**Doc.** `docs/sandbox-asset-cache.md`, for the operator or engineer who
meets the cache for the first time: what lives under `/var/lib/ate` and
why it must share a mount; how an actor gets its assets, from the
runtime the request names through the index, the cache and the hard
links, and what Run, Restore and Checkpoint each do; how the worker
sandbox syncer decides which runtimes a node needs, prewarms and pins
the defaults, and prunes; the eviction rules and the six flags; the
three metrics and what a shortfall means; a runbook for a node over
budget; and what is deliberately left out. Linked from the README's doc
list, the node supervisor section of the architecture page, and the
metrics paragraph of the observability guide.

The page describes the request as naming the runtime the worker
registered, which is agent-substrate#2788's behavior rather than this stack's. It
documents the state once both land rather than the interim.

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