Stacked on #544 (feature/iaf-backtest-store), which is stacked on #543 → #537. Merge order: #537 → #543 → #544 → this PR.

Second slice of Phase 3 of epic #540 — adds a real tiered storage implementation that delivers the cross-run analytics value while keeping the canonical .iafbt bundle as the source of truth (full Tier-2-as-canonical lands in Phase 3d).

What's in this PR

Storage layout

<root>/
  index.sqlite                                  ← Tier-1, always in sync
  bundles/<handle>.iafbt                        ← canonical bytes
  parquet/
    portfolio_snapshots/run_id=<handle>/part-0.parquet   ← Tier-2 hive-partitioned
    trades/run_id=<handle>/part-0.parquet                ← Tier-2
    orders/run_id=<handle>/part-0.parquet                ← Tier-2

The bundle is the canonical representation. Tier-1 and Tier-2 are derived, eagerly maintained, and best-effort: a malformed sidecar never blocks a write or read against the bundle. This keeps Phase 3b's invariants simple and trivially preserves byte-identical Backtest.save_bundle / Backtest.open round-trips today. Byte-identical Tier-2 → Backtest reassembly (no bundle on the read path) is Phase 3d.

Modules

• decompose.py — Backtest → flat record lists for snapshots / trades / orders, adding run_id and window_name columns so downstream tools group cleanly across walk-forward windows. Extension point for metric_series and any future kind is the DATASETS tuple.
• LocalTieredStore — implements BacktestStore + SupportsCopyFrom. write() saves the bundle, upserts the Tier-1 row, and writes hive-partitioned Parquet sidecars per dataset. delete() removes all three tiers. iter_index_rows() serves from SQLite directly. rebuild_index() recreates Tier-1 from the bundles (useful after a software upgrade that adds new index columns).

Cross-run analytics

store = LocalTieredStore("~/.iaf/store")
ds = store.scan("portfolio_snapshots")  # pyarrow.dataset.Dataset
df = ds.to_table(filter=ds.field("run_id") == "top_run_x").to_pandas()

Or directly from DuckDB:

SELECT run_id, total_value
FROM read_parquet('store/parquet/portfolio_snapshots/**/*.parquet',
                  hive_partitioning=True)
WHERE run_id IN (SELECT bundle_path FROM read_csv('top20.csv'));

Partition pruning on run_id is automatic — DuckDB scans only the relevant directories.

Tests

15 new tests, all passing:

• Protocol + SupportsCopyFrom conformance.
• Three-tier layout on write().
• Handle normalisation (.iafbt suffix stripped).
• Round-trip preserves algorithm_id; summary_only honoured.
• Tier-1 always in sync: iter_index_rows after writes; delete removes all tiers; __len__ uses the index.
• Tier-2 cross-run scan returns an Arrow Dataset with run_id column; unknown dataset name raises.
• copy_from from LocalDirStore (interop with Phase 3a).
• rebuild_index() recreates Tier-1 from bundles.
• Missing handles raise StoreHandleNotFoundError.
• Synthetic-records test asserts hive partitions are written and that scan() returns the expected rows + columns when records are present.

Targeted suite (tests/services/backtest_store/ + tests/services/backtest_index/ + tests/cli/): 101 / 101 passing.

What's still coming in Phase 3