[phase-31 2/4] Compaction metadata types

[g-talbot]

Summary

Extend MetricsSplitMetadata with 6 compaction fields and update PostgreSQL model types (Phase 31 Metadata Foundation, PR 2 of 4).

Stacks on gtt/phase-31-sort-schema (PR #6242).

What's included

split/metadata.rs:

• 6 new fields on MetricsSplitMetadata: window_start, window_duration_secs, sort_fields, num_merge_ops, row_keys_proto, zonemap_regexes
• parquet_files: Vec<String> field (replaces single-file assumption)
• Builder methods for all new fields
• TW-1 and TW-2 debug_assert! invariants from ADR-003
• Backward-compat serde defaults for pre-Phase-31 JSON

split/postgres.rs:

• Compaction columns added to InsertableMetricsSplit and PgMetricsSplit
• from_metadata() populates new SQL columns
• SS-5 debug assertions for JSON/SQL column consistency

schema/fields.rs:

• ParquetField::from_name() for name-based field lookup (used by sort field resolver)

Verification

• cargo build -p quickwit-parquet-engine ✅
• cargo test -p quickwit-parquet-engine -- split::metadata split::postgres ✅ (11 tests)
• cargo clippy -p quickwit-parquet-engine --all-features --tests ✅

Test plan

• Pre-Phase-31 JSON backward compatibility test
• Round-trip test with all compaction fields
• skip_serializing_if test for optional fields
• Postgres model round-trip test with compaction columns
• Clippy clean

(Replaces #6243 which was accidentally auto-merged during a rebase)

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