feat(contract): facet_schema — classid-selected 6x2 | 4x3 | 2x48 facet readings

[claude]

Summary

Adds lance_graph_contract::facet_schema — a small, purely additive module
that formalizes the classid-selected format of a FacetCascade's 12 payload
bytes. 96 bits tiles three ways, all exact, all re-tilings of the same
bytes (no re-encoding, no byte moved):

schema	tiling	precedent
TierCascade	6 × (8:8) (part_of:is_a)	the existing FacetCascade::tiers / hi_chain / lo_chain
SpoTriplet	4 × (8:8:8) (subject:predicate:object)	the ruff_spo_* triple corpus
Pair48	2 × 48-bit	already present: helix Signed360 and cam_pq [u8; 6] are both 48-bit

New API on FacetCascade (all const): payload(), schema(),
as_triplets(), as_pair48(), from_triplets(), from_pair48(); plus the
FacetSchema enum + FacetSchema::of_classid().

Scope / safety

• Additive only. New module + one pub mod line; no existing type, method,
  or layout changed.
• Does NOT touch the value slab. The operator-LOCKED 480-byte NodeRow.value
  / ValueTenant / soa-value-tenant-migration-v1/v2 surface is untouched —
  this is purely the 16-byte key facet.
• Default-preserving. FacetSchema::TierCascade == 0, so every existing
  facet keeps its 6 × (8:8) meaning.
• One flagged provisional: the exact bit-position of the schema sub-field
  within the {domain}{schema} facet_classid is marked pending ratification
  in FacetSchema::of_classid (defaults to TierCascade); the explicit
  as_*/from_* accessors need no convention.

Test Plan

#[cfg(test)] round-trip tests included (triplet/pair48 round-trip + classid
preserved; payload is the 12 bytes after the classid; both re-tilings are the
same bytes; schema defaults to TierCascade). Relies on lance-graph CI for
cargo test / clippy / fmt (authored via API without a local build).

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https://claude.ai/code/session_01Mwq1QKpw4zRd6oaGRoJhF2

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