feat: partial-shard write support in ShardingCodec

Adds _encode_partial_sync and _decode_partial_sync to ShardingCodec.
For fixed-size inner codec chains and stores that implement
SupportsSetRange, partial writes patch individual inner-chunk slots
in-place instead of rewriting the whole shard:

  - Reads existing shard index (one byte-range get).
  - For each affected inner chunk: decodes the slot, merges the new
    region, re-encodes.
  - Writes each modified slot at its deterministic byte offset, then
    rewrites just the index.

For variable-size inner codecs (e.g. with compression) or stores that
don't support byte-range writes, falls through to a full-shard rewrite
matching BatchedCodecPipeline semantics.

The partial-decode path computes a ReadPlan from the shard index and
issues one byte-range get per overlapping chunk, decoding only what
the read selection touches.

Both paths are dispatched from SyncCodecPipeline via the existing
supports_partial_decode / supports_partial_encode protocol checks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-v-b