Core SHACL formalism IR (Layer 1).

This crate is the formal heart of the engine: the path algebra π, the shape grammar φ, value types T, selectors, schemas, node expressions, and the rule skeleton. It is the SHACL fragment of arXiv:2502.01295 specialized to RDF (see docs/00-formalism.md), plus the extensions catalogued in docs/01-gap-analysis.md.

Nothing here depends on a graph store or a parser; downstream crates (shifty-parse, shifty-opt, shifty-engine) build on these types.

Layout

• [term] — RDF term aliases and node-kind sets.
• [path] — the path algebra π (Def. 3).
• [value_type] — value types T / test(τ) facets.
• [shape] — the shape grammar φ (Def. 4) and the cyclic-capable arena.
• [selector] — selectors (Def. 5).
• [expr] — SHACL-AF node expressions (gap-analysis AF-E).
• [rule] — SHACL-AF rule skeleton (gap-analysis AF-R).
• [sparql] — opaque SPARQL escape-hatch leaves.
• [schema] — a SHACL schema S = { (sel, φ) } plus rules.