fix(dicom): preserve slices across incremental imports

[PaulHax]

Builds on #934, which is now merged.

Summary

Importing more files for an already open DICOM series reallocates and re-sorts the volume. The old path could lose loaded pixels, write an in-flight decode into the wrong slice, or leave newly added chunks unloaded.

This change preserves existing slices across reallocation and keeps decode results tied to the current buffer and chunk order.

Import flow

incoming chunks
    |
    v
load metadata and remove duplicates
    |
    +-- no new chunks -------------------> no-op
    |
    v
serialize additions and sort the combined chunk list
    |
    v
allocate the new buffer
    |
    v
redecode loaded chunks and start new chunks
    |
    +-- stale decode result -------------> discard
    +-- current decode fails ------------> mark that chunk errored
    +-- current decode succeeds ---------> write its current slice

What changed

Reallocation

Before:

1. New chunks were appended.
2. The volume was sorted and reallocated.
3. Previously loaded slices became zeroes, but their status stayed Loaded.
4. A decode that started before the sort could write using its old slice index.

Now:

1. Chunk additions are serialized so overlapping additions cannot drop chunks.
2. Loaded chunks are marked Loading and decoded into the new buffer.
3. A decode resolves its slice from the current chunk order.
4. Results from an older allocation generation are discarded.
5. An already registered image starts loading chunks added by a re-import.

Tests

Coverage includes serialized additions, sorted slice placement, stale decode success and failure, loading state during reallocation, and starting newly added chunks.

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