feat(storage): CompactStore v5 mapped graph structures (G-EM0.2)

[jarmen423]

Summary

Implements G-EM0.2 — bounded graph metadata and mapped proportional structures — on top of G-EM0.1 container mmap ownership.

• CompactStore payload v5 with checked 64-byte header + 48-byte segment directory (R0 contract)
• Mapped views for CSR arrays, string dictionaries, sorted ID lookups (O(log N)/O(log E)), and column bodies
• Split memory accounting (mapped_payload_index_bytes, anonymous_owner_schema_bytes, proportional must be 0)
• mapped_graph_tests unit suite + compact_store_readonly_graph_parity at two graph sizes
• v1–v4 compatibility readers preserved; default writer emits v5

Verification

export TMPDIR=/data/tmp
export CARGO_TARGET_DIR=/data/cargo-targets/jfrie-grafeo-em0-2/target
export RUSTC_WRAPPER=sccache SCCACHE_DIR=/data/sccache

cargo test -p grafeo-core --lib --features compact-store mapped_graph_tests -- --nocapture
cargo test -p grafeo-core --lib --features compact-store graph::compact::section:: -- --nocapture
cargo test -p grafeo-engine --features compact-store \
  --test compact_store_direct_mmap \
  --test compact_store_readonly_graph_parity \
  --test compact_store_allocation_inventory \
  --test compact_store_integration \
  --test compact_store_large_string_persistence -- --nocapture

All of the above PASS in this leaf session.

Notes for parent

• Packet base is G-EM0.1 head 7ae5f217… (feature/g-em0-1-direct-mapped-container).
• Do not merge to main from this leaf; parent owns integration after review.
• Zone-map table/block segments (kinds 18–19) are optional and not yet fully populated on the v5 writer; property/CSR/ID paths are the G-EM0.2 exit-gate surface.
• Concurrent writable open of the same path can invalidate an existing RO mapping (file-backed strings); the direct_mmap test now uses a byte-identical copy for the LegacyEager check.