Follow-ups: Rosetta x86 / multi-platform cleanup (deferred from #279, #54)

[rgarcia]

Deferred, non-blocking cleanup opportunities identified during review of the Rosetta x86 / multi-platform PRs (#279, kernel/hypeman-cli#54). None affect behavior; they were intentionally left out to keep those PRs reviewable. Two are marked inline with TODO(followup) and link here.

hypeman (server)

• Unify Resolve / ResolveForPlatform (lib/images/reference.go) — near-duplicate methods differing only by platform-aware vs platform-less manifest inspection; collapse behind one path. (inline TODO(followup))
• createAndQueueImage re-resolves the platform (lib/images/manager.go) — CreateImage already parsed/validated the platform; pass it in instead of calling resolveRequestPlatform again, which also removes the effectively-unreachable error branch. (inline TODO(followup))
• ResolvedRef.DigestRef() helper — replace hand-assembled repo + "@" + digest strings with a method on ResolvedRef.
• conversionFailedErr(msg) helper — extract the 3 duplicated WaitForReady failure-message branches in lib/images/manager.go.
• Single source of truth for the rate-limit user message — one package const, or fold the actionable hint into ErrRateLimited and use err.Error() in the handlers (cmd/api/api/{images,instances}.go).
• Tests: assertFailsFastNotFound helper for the two structurally identical fail-fast restore tests; a generic captureManager[Req] to collapse the 4 near-identical capture mocks (cmd/api/api/instances_test.go).

hypeman-cli

• Drop the platform WithJSONSet / RawJSON workarounds once the hypeman-go SDK exposes a typed platform field — inspect/image list/run/pull can then read the typed field directly. Marked inline with TODO(sdk-bump:...).

Filed from the reviewer cleanup pass; see the discussion on #279.