The shared cross-domain substrate for the Fuaran family — the genericity-extracted spine that the UI, Calc, Documents, CAD, Office (and future) domain tiers consume as a peer dependency, so each domain stops re-implementing the same op-stream / op-algebra / tree / wire / validator / artifact-function machinery five times over.

Apache-2.0. FSharp.Core + Fable.Core only (Fable.Core is a compile-time dependency for the dual .NET/Fable pipeline — no runtime behaviour rides on it). No domain dependency, no native dependency, no host dependency. Every package is a library of generic functions over domain-witness records — the core owns no base node type. Each domain's closed NodeKind DU stays sovereign and exhaustively matched — closed unions + exhaustive total matching are the load-bearing F# constraint the whole pattern rests on.

This repo is the realisation of the rule-of-three extraction: the substrate is extracted only now, against five shipped artifact-witness spines (UI, Calc, Documents, CAD, Office), with the string-vs-Guid identity axis resolved as a witness parameter rather than guessed.

Dependency order: Tree → Ops → (OpStream → OpStream.Dag; Wire standalone); Column over Wire → DataFrame over Column; Validator over Tree + Column; Function over Tree/Ops/Wire; Column.Ops over Column + DataFrame + OpStream; Query over Column + Function; Conformance over all of the above.

The core never sees a concrete NodeKind. A domain supplies a small record of functions (the witness) and the generic functions operate through it:

let nodew : NodeWitness<MyNode, MyId> =
    { Id = fun n -> n.Id
      KindTag = fun n -> n.Kind
      Children = fun n -> n.Children
      ReplaceChildren = fun n cs -> { n with Children = cs } }

// now the whole skeleton op algebra works over MyNode, with no core change:
Ops.apply nodew idw (InsertChild(parentId, 0, child)) tree

The one genuine cross-domain divergence — id representation — is a witness too: IdWitness<'Id> = { ToString; OfString; Equals }. Doc/Calc keep human-meaningful string ids; UI/Music keep Guids; both over one Core.Tree.

A saved typed tree behaves as a function of its declared holes. The contract bakes in:

1. Totality — bounded iteration only; an unbounded RepeatHole is rejected, never run.
2. Hygiene — holes bind by absolute lexical address (id-path), never bare name, so composition cannot capture.
3. Effect signature — a mandatory two-axis effect/determinism class, joined componentwise through compose (pure ∘ clock = clock).

./run.ps1            # format + build + test
./verify.ps1         # format-check + build + Fable-compile gate + test (the green gate)
dotnet build Fuaran.Core.slnx
dotnet run --project tests/Fuaran.Core.Tests

./verify.ps1 includes a Fable-compile gate (Phase 54): tests/fable-smoke/ references every public package and is compiled with dotnet fable, so the "Fable-clean on encode and decode" claim is enforced in-repo, not discovered downstream — a package that stops compiling under Fable fails the gate.

398 conformance tests exercise every layer against an in-repo reference witness (a tiny string-id domain) — proving the generics work without depending on any domain workspace. Domain adoption (re-expressing each domain's machinery over Fuaran.Core.*) is deliberately out of scope here; it lands on each domain workspace's own roadmap.

Re-expressing a domain spine over Fuaran.Core.*? Start at docs/ADOPTION.md — the four-witness recipe + the caveats a real adoption surfaced — with the runnable template at samples/adoption.

Pre-1.0 — the released version is single-sourced from <Version> in Directory.Build.props. The witness-record contracts (especially IdWitness and NodeWitness) are the stability-critical surfaces — see STABILITY.md. Design log in DECISIONS.md.