Function probeOfflineStorageBudget

Read the origin's real storage estimate and derive a conservative logical budget, or report explicitly that the platform cannot estimate.

Performs no network I/O, writes nothing, and never calls StorageManager.persist(). persisted() is read when the platform offers it because persisted state changes the eviction risk of every cached region; a platform that does not offer it yields persistence: "unknown" rather than an assumption.

Derivation is deterministic integer arithmetic over the reported values:

remaining = max(0, quota - usage)
reserve   = min(remaining, max(minimumReserveBytes, floor(remaining * headroomRatio)))
budget    = remaining - reserve
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so the budget can never exceed the platform-reported remaining quota, and a nearly full origin yields 0 instead of a number that cannot be honoured.