fix(runtime): prefer host type flags over mode bits in Stats

[wasaybuilds]

Closes #1839.

• Every host stat payload already carries isDirectory and isSymbolicLink (javascript_sync_rpc_stat_value, javascript_sync_rpc_host_stat_value, HostStat::to_value), but Stats derived all seven type predicates from the mode bits alone. Dirent, right below it, already trusts its explicit boolean.
• The type is now resolved once in the constructor from the host flags, falling back to the mode bits when a backend omits them. A backend that sends no hints behaves exactly as it does today.
• Resolving a single format also keeps the predicates mutually exclusive, which mode bits plus a contradicting hint would not.

Behaviour of the real Stats class, before and after:

case                                   BEFORE           AFTER
--------------------------------------------------------------------------
regular file, host agrees              isFile           isFile
STALE CACHE: dir bit on a real file    isDirectory      isFile          <-- changed
real directory                         isDirectory      isDirectory
symlink                                isSymbolicLink   isSymbolicLink
STALE CACHE: file bit on a real dir    isFile           isDirectory     <-- changed
legacy backend, no hints (dir)         isDirectory      isDirectory
legacy backend, no hints (file)        isFile           isFile
fifo, hints both false                 isFIFO           isFIFO

Only the two cases where the mode bits contradict the host change, and both change to the host's answer. Row 2 is the reported failure: the Claude CLI's Edit tool seeing EISDIR on a regular file.

_typeFormat is defined non-enumerable, so mode is untouched and JSON.stringify(stats) / Object.keys(stats) are unchanged.

Same caveat as #1908 — no committed test, because the guest fs suites need a VM I can't build locally. Glad to add one wherever you'd want it.