PR: storage: Phase 3 — AsyncFihIo, FlushCapable, chain flush, ColdStorage trait, Time Machine tests #85

[metasmile]

PR: storage: Phase 3 — AsyncFihIo, FlushCapable, chain flush, ColdStorage trait, Time Machine tests #85

Summary

Phase 3 completes the storage abstraction stack for NativeFihStorage. It consolidates the FIH storage prototype around a single AsyncFihIo trait, adds chain-based incremental flush (cursor_{ts}.chain), implements ColdStorage + ScanCapable + TimeRangeCapable for DualStorage compatibility, and validates the full StateSpace as a 4D time-travelable storage (Fact × Intent × Hint × time).

Changes

File	Change
storage/sim/src/io.rs	AsyncFihIo trait with BlockingFihIo wrapper; remove sync FihIo
storage/sim/src/sim_io.rs	SimFihIo: deterministic in-memory IO, failure injection
storage/sim/src/fs_io.rs	FsFihIo: filesystem-backed IO, WASI compatible
storage/sim/src/intent_status.rs	IntentStatus enum state machine (try_claim/heartbeat/conclude)
storage/sim/src/record.rs	FactRecord, IntentRecord, HintRecord with postcard serialization
storage/sim/src/index.rs	TimeIndex: Vec+(partition_point), first_ts/last_ts
storage/sim/src/session.rs	FihSession: hydrate/flush lifecycle wrapper
storage/sim/src/store.rs	NativeFihStorage: 8 trait impls, type aliases, is_hotmemory_enabled, ColdStorage + ScanCapable + TimeRangeCapable + chain flush
storage/sim/src/main.rs	15-check verification runner (cargo run)
storage/sim/tests/concurrency.rs	6 concurrent access tests
storage/sim/tests/gaps.rs	6 gap coverage tests
storage/sim/tests/time_filter.rs	6 deterministic time-index filter tests
storage/sim/tests/foundational_scenarios.rs	4 SSCCS document scenario tests
storage/sim/tests/gateway_scenarios.rs	1 MockGateway round-trip test
storage/sim/tests/time_machine.rs	8 StateSpace time-travel tests (NEW)
storage/sim/tests/common/mod.rs	FakeClock + Fact/Intent helpers

Key Design Decisions

• Chain flush: Each flush writes cursor_{ts}.chain containing only delta records since the previous cursor. File count = O(flush count), not O(record count). Enables incremental replay.
• Postcard over bincode: bincode is a discontinued project. Postcard provides the same zero-copy deserialization semantics and is actively maintained.
• is_hotmemory_enabled: When NativeFihStorage serves as ColdStorage (DualStorage cold half), in-memory caches are skipped — all reads go directly to IO, saving memory.
• Type aliases: FactCache, IntentCache, HintCache, RefCounts, OriginIndex isolate the concrete map type. Switching from HashMap to BTreeMap requires only 5 alias lines.

4D StateSpace (Storage Paradigm)

nexus storage is not a graph database. It is a 4D StateSpace:

Fact axis (immutable observations)    ×
Intent axis (active operations)       ×
Hint axis (ephemeral signals)         ×
Time axis (delta chain, time-travel)

Coordinate traversal (moving along axes, projecting across dimensions) IS the query mechanism. Cypher translation is optional syntactic sugar — the native API (FilterCapable + TimeIndex + by_from_fact) already provides the full coordinate algebra.

Validation

Check	Result
cargo test	70/70 pass (62 + 8 time_machine)
cargo run	15/15 pass
cargo clippy -- -D warnings	0 warnings
cargo build --target wasm32-unknown-unknown	OK
run.sh --core (fmt + clippy + test)	All checks passed

Follow-up Issues

• #87 NativeFihStorage: by_from_fact reverse index — Core: Add O(1) reverse lookup to find all Intents referencing a given Fact. Enables cross-axis traversal and prepares for cursor-driven chain replay without full cache rebuild.
• #88 FihExport / FihImport traits — Core: Bundle full StateSpace (chain files + blob store) into a single portable archive (.fihbundle) for storage-agnostic migration.
• #90 R2-native CfFihIo adapter — Infrastructure: AsyncFihIo over Cloudflare R2, enabling NativeFihStorage on Workers WASM.