* fix(volume): reopen .idx writable after MarkVolumeWritable
When .vif has ReadOnly=true, load() opens .idx as O_RDONLY and builds a
SortedFileNeedleMap whose Put returns os.ErrInvalid. MarkVolumeWritable
only flipped noWriteOrDelete back to false and rewrote .vif, so writes
still failed at v.nm.Put. Reopen .idx in O_RDWR and rebuild v.nm in its
writable form (in-memory or leveldb small/medium/large) before flipping
the flag.
Mirror the same fix in seaweed-volume: the Rust load path leaves
CompactNeedleMap/RedbNeedleMap with no idx_file writer when the volume
boots read-only, so post-MarkVolumeWritable puts silently succeeded
in-memory only and were lost on the next restart. set_writable now
reattaches an append-mode writer when one is missing.
* fix(volume): keep old needle map until replacement is built; defer writable flag
Go: build the writable needle map into a local before swapping. A
construction failure now leaves v.nm pointing at the original
SortedFileNeedleMap so MarkVolumeWritable can roll back, instead of
stranding the volume with v.nm == nil.
Rust: attach the .idx writer before flipping no_write_or_delete to
false. A transient open/metadata failure used to leave the volume
marked writable with no writer attached, and subsequent puts would
silently skip the on-disk append.
