* fix(redis3): prevent filer crash from inconsistent skiplist ends
DeleteByKey updated the two ends asymmetrically: the start side decided
whether to clear StartLevels[index] by comparing a cached reference key,
while the end side cleared EndLevels[index] structurally. The redis3
ItemList re-keys a node while keeping its id, so that cached key drifts.
When such a node was the only one at a level and got deleted, the stale
key comparison left StartLevels dangling while EndLevels was cleared to
nil. The next InsertByKey then dereferenced a nil EndLevels[0] and took
down the whole filer during a rename or delete.
Match the deleted node by its unique id so both ends stay consistent,
and guard each end in InsertByKey so an already-corrupted skiplist
persisted in Redis self-heals instead of crashing on load.
* fix(redis3): propagate errors from WriteName node split
The case 2.3 split path returned nil instead of the error in seven
branches. Because the split runs a multi-step sequence (DeleteByKey on
the skiplist, ItemAdd, redis range-delete, ItemAdd), a swallowed failure
let WriteName report success while the skiplist was half-updated, which
the caller then persisted - silently corrupting the directory listing
and setting up the very inconsistent-ends state that crashes the filer.
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