feat(realtime): support spilling sealed segments

[lxy-9602]

Purpose

Linked issue: #158

Add opt-in disk spill support for the default real-time store to reduce memory retained by sealed segments.

• Spill each sealed segment to a local Arrow IPC file.
• Release in-memory batches after spill while keeping pinned views readable.
• Support spilled query and commit readers for append-only and PK tables.
• Delete spill files after their last reference is released.
• Reuse common Arrow IPC utilities in the existing merge-tree spill path.
• Support configurable read batch sizes for real-time queries by slicing large in-memory and spilled batches.

Tests

Added unit and integration coverage for:

• Spill configuration and validation.
• Memory and spilled reads.
• Append-only and PK deletion-vector commit/read flows.
• Spill-file lifetime during pinned reads and reclaim.
• PK reads before and after reclaim.
• Read batch slicing.

API and Format

Adds realtime.spill-enabled and spill context fields.

No persistent table format change. Spill files are temporary local Arrow IPC files.

Documentation

Generative AI tooling

Generated-by: OpenAI Codex (GPT-5)

[zjw1111]

One resource-management issue in the PK spill read path.

[zjw1111]

+1

[SteNicholas]

LGTM.