[core] Parse primary-key vector index source metadata (_SOURCE_META)

[JunRuiLee]

Purpose

Linked issue: part of #514 — PR 1 of 5 (does not close the issue)

This is the first, read-only slice of primary-key vector (bucket-local ANN) search support. It mirrors Apache Paimon (Java) apache/paimon#8549 as of commit a50a36ff8.

Java primary-key vector indexes append a nullable _SOURCE_META blob to GlobalIndexMeta listing the ordered source data files and row counts of a bucket-local ANN payload; an ANN ordinal maps to a (data file, physical row position) pair. This PR gives paimon-rust the ability to parse that metadata and resolve an ordinal — the foundation PR 2 (bucket search) and PR 3 (physical-position read) build on.

Read/parse only. Out of scope for this PR (tracked in #514): ANN search, manifest selection, physical-row materialization, index building, and engine SQL entry points.

Brief change log

• Add a nullable source_meta: Option<Vec<u8>> field to GlobalIndexMeta (serde _SOURCE_META, serde_bytes), matching Java's 6th GlobalIndexMeta field.
• New spec/pk_vector_source.rs: PkVectorSourceFile / PkVectorSourceMeta, a byte-for-byte parser for Java's PkVectorSourceMeta frame (version 1, count, writeUTF filenames, writeLong row counts, trailing-bytes check), and an ordinal-only resolve(ordinal) -> (file, position) prefix-scan resolver.
• Decode _SOURCE_META through the hand-written Avro index-manifest decoder, schema-guarded: a pre-#8549 5-field _GLOBAL_INDEX writer schema is detected from the file's writer schema and the 6th field is not read, so old manifests never misalign the byte stream.
• The _SOURCE_META inner blob is Java DataOutput (big-endian ints/longs, modified UTF-8), parsed independently of the outer Avro layer; parsing helpers are module-private (no shared common/ extraction in this PR).

Tests

All in spec/pk_vector_source.rs and spec/index_manifest.rs (22 tests; full cargo test -p paimon green):

• Frame parsing: single/multi source file; rejects bad version, zero count, trailing bytes, truncated input, negative row count, empty list.
• Java modified UTF-8: ASCII/2-byte/3-byte/supplementary round-trips; C0 80 → NUL; raw 0x00 accepted (Java readUTF parity); truncated/bad-continuation and lone-surrogate rejected.
• Resolver: single-file, multi-file prefix-sum boundaries, negative/out-of-range ordinal errors, checked_add overflow branch.
• Avro: new-format _SOURCE_META round-trips (Some/None); legacy 5-field _GLOBAL_INDEX decodes without misalignment and yields source_meta = None.

API and Format

• No storage-format change originated here: this reads the _SOURCE_META format defined by Java [core] Add primary-key vector index foundation paimon#8549. _SOURCE_META is a nullable trailing field, so existing index manifests remain readable (verified by the legacy-schema test).
• Public API additions only (PkVectorSourceFile, PkVectorSourceMeta); no existing API changed.

Documentation

No user-facing documentation changes.

[JingsongLi]

+1