Testing architecture for the standard library

[LesterEvSe]

Problem

simf/lib/ is the product; everything else is scaffolding. Today that scaffolding uses one shape for every feature — a per-feature program with witness dispatch plus per-feature Rust plumbing — but the wishlist features differ in what context they need to run:

Feature	Needs	Category
and/or/not, EC point_to_gej, decimals, identities, deep_eq, is_some/is_none	known input → known output	Vector
checked/safe arithmetic	randomized inputs + failure injection	Property
u256 store/load, Merkle tools	program storage state	Stateful
asset/amount utils, script-hash utils, OP_RETURN, covenants, relative timelocks	a crafted transaction	Scenario

The bulk of the wishlist is Scenario, which has no shared support today.

Proposal

Standardize the universal piece (spend plumbing), then pick the lightest category per feature. Build order:

1. Shared helpers — extract the duplicated fund / spend / broadcast / assert block. Everything below builds on it.
2. Vector tests (default) — self-asserting main(), no witness; Rust side is one call to the helpers. A generic runner enumerates a registry of should-succeed programs. Prove out by converting logical and ct/point.
3. Scenario tests (main future investment) — reusable transaction-scenario builder (inputs/outputs, assets, sequence/version, OP_RETURN, storage) plus a thin contract entrypoint per function. Unblocks covenants, assets, script hashes, timelocks.
   Property tests already exist and stay as-is.

Layout

simf/
  lib/
  logical.simf  ct_point.simf  uint.simf  ...   # test entrypoints, mirroring lib/
tests/
  common/                                        # shared helpers + scenario builder
  logical.rs  ct_point.rs  uint.rs  ...

Rename *_mock.simf → *_test.simf; these are entrypoints, not mocks.

Entrypoints can't live in simf/tests/. A contract resolves modules (crate::lib::...) relative to its main file's directory, so a main under simf/tests/ can't see simf/lib/. Decoupling source root from entrypoint location needs separate Simplex/SimplicityHL work — out of scope here.

Future: enum dispatch

Witness-driven contracts currently select a function via numeric witness::FUNCTION_INDEX with one match if_test_this_function(N, ...) block per case. Once PR #336 lands, indices become named variants and dispatch collapses to a single match with one arm per case. Readability only.

[LesterEvSe]

@Hrom131 @aritkulova @apoelstra glad to hear you have been thinking about this