feat(storage): read-only direct CompactStore container mmap (G-EM0.1)

[jarmen423]

Summary

Implements G-EM0.1 — read-only direct mapped-container ownership for CompactStore reopen.

• Lifetime-safe MmapSection → Bytes::from_owner bridge (into_bytes)
• Read-only open uses mmap_section + deserialize_from_mapped_bytes (no read_section_data / full-section Vec copy)
• Query-visible CompactBacking::ContainerMmap { artifact_id, payload_version, mapped_bytes }
• Mapping retained on CompactStore until last reader drains
• Fail-closed StorageError::DirectMmapUnavailable for encrypted / non-mmap-able / empty sections
• Writable reopens report LegacyEager (not Milestone R evidence)

Packet

• Packet: G-EM0.1
• Source base: 06b318e90e96ecb73d8278a6e02c84c6a4d0cece (integration/g-em0-1-base-f0-r0, accepted F0 + R0)
• Head: see branch tip after push
• Handoff: /home/jfrie/agent-memory-hosted/.hermes/handoffs/grafeo-disk-backed/G-EM0.1.md

Verification

cargo test -p grafeo-storage --lib --features grafeo-file mmap_ -- --nocapture
cargo test -p grafeo-storage --lib --features grafeo-file bytes_views_retain -- --nocapture
cargo test -p grafeo-engine --features compact-store --test compact_store_direct_mmap -- --nocapture
cargo test -p grafeo-engine --features compact-store \
  --test compact_store_integration \
  --test compact_store_large_string_persistence \
  --test compact_store_allocation_inventory -- --nocapture
cargo test -p grafeo-core --lib --features compact-store graph::compact::section:: -- --nocapture

All green. Pre-existing: cargo clippy -D warnings fails on base missing_errors_doc in buffer/manager.rs (unrelated).

Residual (explicit non-claims)

• G-EM0.2 still owns v5 mapped proportional graph structures; v4 mapped open still rebuilds some heap dictionaries (accounted separately from file-backed mapped_bytes).
• Leaf does not merge this PR; parent integration only.