* fix(content): report database faults from the availability check (#239)

IsAvailable and AvailableByUUID return a bool, so a busy, closed or I/O-failed database read as "content missing". Where missing content is tolerated on purpose, that swallowed the fault: since #231, CreateCheckout leaves vfile empty for a partly synced tip, and a fault during resolveFiles' availability check took the same path, so Create succeeded with an empty checkout.

The chain walk moves to checkAvailable, which returns an error for a fault and a plain false for an unknown rid, a phantom, a cycle or an over-long chain. New CheckAvailableByUUID exposes that distinction, and resolveFiles uses it, so a fault fails LoadVFile with its own error instead of errMissingContent. IsAvailable and AvailableByUUID keep their signatures and their fail-closed reading of a fault; their ~30 callers treat "not available" as a failure already.

* docs(changelog): note the availability fault fix (#239)

* fix(content): NULL size is missing content; don't cache faults (#239)

checkAvailable scanned blob.size into an int64, so a NULL size (legal in the schema) became a hard error in resolveFiles where it used to read as missing content. Both walks now read a NULL size as a phantom.

AvailabilityCache memoized a walk's verdict for every node it visited, including a false that came from a database fault, so one transient fault stayed wrong for the rest of the crosslink sweep. A faulted walk now returns false without being remembered; an unknown rid is still a verdict and is cached as before. check-in: 4b14509374 user: [email protected] tags: trunk