Reduced entity kill data by skipping default attributes

[tonyjamesstark]

Each mob kill stores every registered attribute: about 23 entries on 1.21+, almost all at their default value. They make up about 88% of each blob. This PR stores an attribute only when its base value differs from the default, or when it has modifiers.

How much it saves on SQLite: a table leaf page is 4096 bytes and holds at most 4061 bytes of row data. A zombie kill blob is 2036 bytes today, so two rows do not fit and every kill uses a whole page. At 1027 bytes, three rows fit on a page. Measured with dbstat over 20,000 rows:

Zombie kill	Blob	Rows per page	Disk per kill
Current	2036 B	1	4096 B
This PR	1027 B	3	1365 B
This PR after the v26 migration to EntityDataCodec	141 B	26	158 B

This PR is proposed before v26 because it is the only size relief for SQLite and MySQL until then. After v26, it still makes each blob about 1.8 times smaller (250 to 141 bytes). If you prefer to skip default attributes as part of the v26 migration, I can adjust or withdraw it.

Rollback stays exact. New rows carry a marker at kill data index 9. When restore sees the marker, it first resets every attribute of the respawned mob to its default and removes spawn-time modifiers, then applies the stored ones. Index 7 (UUID or null) and index 8 (the kill-location slot for placed entities) keep their meaning. Rows without the marker restore as before. Capture and restore both use AttributeInstance#getDefaultValue(), which exists on every supported version (1.16.5+). It is deprecated since 26.1 but is not scheduled for removal.
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authored and tested with claude

[Intelli]

Thanks! The storage reduction looks worthwhile, but the sparse format needs to preserve correctness across server upgrades before this can merge.

• resetAttributes() currently resets every attribute available on the restore server. For example, a horse logged before 1.20.5 and restored on 26.1 would have its step height, safe fall distance, and fall damage multiplier reset to global defaults of 0.6/3/1 instead of the horse defaults of 1/6/0.5. Those attributes did not exist in the original record, so they should retain their normal spawn values.
• Please make the sparse format identify which attributes were omitted and reconstruct their original baseline values independently of the current server’s defaults. Attributes absent from the original snapshot should remain untouched. An immutable, versioned baseline with explicit storage for attributes outside that baseline could work. Please retain the existing behavior for unmarked records.