feat(store): add key_prefix_from cursor to Query for efficient prefix scans

[royalcala]

Summary

Adds an optional from cursor field to the Query struct that enables B-tree-level seeks during prefix scans.

Motivation

The current key_prefix filter always starts iteration from the beginning of the prefix range. For applications maintaining a cursor position (e.g., syncing event logs), this forces O(n) client-side skipping via offset. With this change, the B-tree seeks directly to the cursor position in O(log n).

Changes

• Add from: Option<Bytes> field to Query and QueryBuilder (store.rs)
• Add key_prefix_from(prefix, from) builder method (store.rs)
• Pass cursor through IndexKind::KeyAuthor to ByKeyBounds::new (util.rs, bounds.rs, query.rs)
• When from is set with a Prefix filter, the lower bound becomes the cursor position instead of the prefix start

Backward Compatibility

Fully backward-compatible. When from is None (default), behavior is identical to current key_prefix. The wire format adds an optional field that defaults to None.

Usage

// Iterate from a known cursor position
let stream = doc.get_many(
    Query::all()
        .sort_by(SortBy::KeyAuthor, SortDirection::Asc)
        .key_prefix_from("evt:", "evt:01JABC...")
        .limit(100)
).await?;