fix(runtime-host): serve one revision-consistent Usage snapshot

## Problem

Desktop now reads Settings > Usage from the selected Runtime Host, but one page load issues independent summary, log, and pricing queries. The wire contract has no snapshot identity or revision, and the Desktop adapter currently accepts a degraded result if repair or Host replacement makes those reads disagree. One rendered page can therefore mix revisions.

## Scope

- Add a bounded opaque snapshot ticket to the Runtime Host Usage query contract.
- Return one authoritative revision with Usage summary and page results.
- Pin a bounded Host repair/read pass to that ticket and return a typed revision-changed error when it cannot be honored.
- Make Desktop retry the whole Usage load a bounded number of times instead of mixing partial revisions.
- Add protocol, coordinator, and Desktop adapter tests.
- Raise RUNTIME_HOST_COMPATIBILITY_EPOCH strictly above the value on main when the wire change lands.

## Non-goals

- Usage UI redesign or coverage-state presentation.
- Pricing mutation semantics.
- Rewriting Runtime Host repair authority.
- Removing the legacy Storage Usage reader.
- Adding another durable Usage authority.

## Acceptance

- Summary, model logs, tool logs, and pricing used by one Desktop load are revision-consistent or the load fails/retries as one unit.
- Retry and snapshot lifetime are bounded.
- A Host replacement cannot silently combine old and new results.
- Compatibility tests cover old/new epoch rejection and the typed revision-change path.

## Related

- #2128 tracks missing/partial/unpriced/aborted coverage and presentation.
- #2641 is the superseded broad implementation branch.

Start by tracing the Runtime Host Usage query contract, the Host repair/read coordinator, and the Desktop adapter mentioned in the scope. Run or add the protocol, coordinator, Desktop adapter, and compatibility tests; done means bounded snapshot and retry paths keep all results on one revision, reject incompatible epochs, and expose the typed revision-change error.

Written by the indexing model from the issue text.