[c++] Add write callbacks with batch completion dispatch

[naivedogger]

Purpose

Linked issue: close #4276

Add C++ write callbacks so applications can handle write completion without maintaining a separate WriteResult waiting queue.

Brief change log

• Add callback overloads for Append, AppendArrowBatch, Upsert, and Delete.
• Register callbacks directly on Rust write batches and dispatch them through shared workers, avoiding per-record async waiting tasks.
• Add coverage for registration races, error propagation, result ordering, and callback lifetime and exceptions.

Tests

• Rust core: 794 tests passed; Rust C++ bindings: 9 tests passed.
• Existing C++ test binary: 4 non-cluster tests passed.
• cargo fmt, cargo clippy, and git diff --check passed.
• No live-cluster tests or full Maven verification for this revision.

API and Format

• Add C++ callback overloads while preserving existing Wait/await result semantics and retry behavior. Shared completion allocation and notification scheduling change.
• Return submission status synchronously and report completion through callbacks. Submission errors do not register callbacks; multi-bucket ArrowBatch writes remain non-atomic.
• Callbacks may execute concurrently and out of order. The completion queue is unbounded, and Flush/destruction do not drain callbacks.
• The existing Rust shutdown-retry issue, delivery deadlines, and durable recovery are outside this PR's scope.
• No wire or storage format changes.

Documentation

Update the C++ API reference with callback examples, compatibility notes, partial-failure semantics, and application responsibilities for memory limits and shutdown.