feat(store): Tier-3 content-addressed OHLCV chunks (epic #540 phase 3c)

[MDUYN]

Phase 3c — Tier-3 content-addressed OHLCV chunks

Stacked on #545 (Phase 3b). Part of epic #540.

This slice unlocks the headline storage win for users who run many
backtests over a shared universe: identical OHLCV is stored exactly
once across the entire store, no matter how many bundles reference it.

What's in the box

LocalTieredStore.write() now routes save_bundle's OHLCV writes to
<root>/ohlcv/ whenever the backtest carries price data. The bundle
envelope still uses the existing content-addressed manifest
({key -> "<sha256>.parquet"}), so old bundles remain readable and
new ones simply share the storage location.

store.open() forwards the same shared directory to open_bundle, so
OHLCV lookups resolve regardless of where the bundle was originally
written.

store.delete() deliberately leaves the ohlcv/ directory alone —
chunks are globally shared. Orphans are reclaimed on demand.

New introspection / GC API

Method	Purpose
iter_ohlcv_hashes()	yields one hash per (handle, key) reference (with duplicates)
ohlcv_referenced_hashes() -> set	deduplicated set across all bundles
ohlcv_stored_hashes() -> set	what's physically on disk under ohlcv/
ohlcv_stats() -> dict	{stored_blobs, stored_bytes, referenced_blobs, orphan_blobs, missing_blobs}
garbage_collect_ohlcv(*, dry_run=False) -> list[str]	reclaim orphans; dry_run=True returns the audit list without deleting

Manifests are decoded straight from the bundle envelope via
_decode_payload — no full Backtest instantiation, so introspection
across thousands of bundles stays cheap.

This is exactly the set-difference primitive that
docs/design/ohlcv-dedup-protocol.md
needs for its POST /ohlcv/negotiate step.

Tests

9 new tests in tests/services/backtest_store/test_local_tiered_store_ohlcv.py:

• No OHLCV → no chunk dir created.
• Identical OHLCV stored once across distinct handles (the dedup invariant).
• Different OHLCV → separate chunks.
• Round-trip via store.open() resolves OHLCV from the shared dir.
• delete() keeps still-referenced chunks; orphans only appear when the last reference is gone.
• garbage_collect_ohlcv(dry_run=True) lists without deleting; the real call removes them.
• iter_ohlcv_hashes() emits per-reference; ohlcv_referenced_hashes() dedups.
• Hash strings are 64-char lowercase hex (matches the upload-protocol spec).

Targeted suite (tests/services/backtest_store/ + tests/services/backtest_index/ + tests/cli/): 110 / 110 passing.

What's left in Phase 3

Slice	Scope
3d	iaf migrate-store --from local-dir --to local-tiered; byte-identical Tier-2 → Backtest reassembly (.iafbt becomes export-only); parameterised pytest fixture running every backtest test against both stores