[core] Support filesystem partitions in SystemRead

[JingsongLi]

Purpose

PyPaimon already exposes filesystem $partitions metadata, including deletion-vector cardinalities, but Table.new_system_read("partitions") was unsupported. Add this missing core implementation so Python can reuse the existing thin SystemRead binding.

Changes

• Read the 12 Java PartitionsTable columns from the resolved manifest snapshot. Files and deletion-vector metadata share one pinned snapshot, including fixed-empty and historical views.
• Share typed partition aggregation with filesystem catalog listing and Table.partition_stats(). Preserve declared total_buckets rather than counting occupied buckets, and resolve file creation times as instants before aggregation.
• Sum deletion-vector cardinalities across buckets, preserve unknown cardinalities as NULL, and skip index-manifest reads when neither projection nor predicates need the count. Normalize each partition through Java's legacy/SQL spec round trip before both display and deletion lookup. Respect default/NULL, binary UTF-8, legacy TIME precision, TIMESTAMP precision, and LTZ DST gap/overlap rules.
• Render typed partition specs with Java casts, default partition names, Java whitespace and UTF-8 rules. Reuse the existing Java floating formatter.
• Convert DataFusion partition epoch timestamps back to system-local TIMESTAMP columns, matching Java and core SystemRead.
• Format BinaryRow timestamp values directly from millisecond/sub-millisecond storage, sharing formatting with schema evolution. This avoids Arrow nanosecond overflow for high-precision timestamps outside 1677–2262.

This covers the filesystem metadata behavior already implemented by Python. Catalog-owned partition audit metadata and the DataFusion catalog-backed provider retain their existing paths. No new public builder or Python binding API is introduced.

Validation

• Core system-table tests under TZ=America/New_York: 39 passed.
• Core partition metadata end-to-end tests: 2 passed.
• DataFusion system-table integration tests under TZ=Asia/Shanghai: 29 passed; SQL/core/files creation timestamps agree.
• Workspace Clippy, Rust 1.98.0, --all-targets --features fulltext,vortex -- -D warnings: passed.
• cargo fmt --all -- --check and git diff --check: passed.
• Companion Python Native partition metadata tests against the rebuilt binding: 27 passed in UTC (4 New York golden cases skipped), 31 passed in America/New_York. Another 72 system-table regressions passed in UTC.
• Original review reproductions now match Java for legacy TIME deletion counts, legacy LTZ display, and non-legacy LTZ overlap deletion keys.

Independent verification completed; actionable findings were fixed with regression tests.

Companion PyPaimon PR: apache/paimon#10605 (draft until this PR is merged).

[leaves12138]

LGTM