feat: expose point-query primitives in paimon

[JingsongLi]

Purpose

Expose the minimal Paimon core primitives needed by snapshot-consistent point-query consumers, independently of any HTTP service implementation.

Linked issue: none yet.

Brief change log

• Add public Blob descriptor version and range validation APIs while reusing the same validation for payload reads.
• Add a typed SnapshotNotExist error and a strict time-travel table copy API that propagates selector failures.
• Expose the lower-bound sum of known data-file bytes on Plan and ScanTrace for query budget enforcement.

This PR intentionally contains only changes under crates/paimon. The query service is tracked separately in #738.

Tests

• cargo test -p paimon --lib (2376 passed, 2 ignored)
• cargo test -p paimon spec::blob_descriptor::tests --lib (11 passed)
• cargo test -p paimon table::source::tests --lib (51 passed)
• cargo clippy -p paimon --lib -- -D warnings
• cargo fmt --all -- --check

API and Format

Adds public Rust APIs for strict snapshot selection, typed missing-snapshot errors, Blob descriptor validation, and planned data-file byte statistics. No storage or serialization format changes.

Documentation

Public APIs include rustdoc explaining strict selection, descriptor validation, and lower-bound byte accounting.

[leaves12138]

LGTM. The overflow-safe byte accounting and regression test look good.