feat: expose the program idl as a flake output

[0xgleb]

🦋 GitButler workspace — open PRs in data-cartel/fund

• docs: spec create_fund and Anchor essentials #6
• feat: add the create_fund instruction #7
• feat: add the deposit instruction #8 (stacked on feat: add the create_fund instruction #7)
• docs: ADR for donation-resistant share pricing #10
• docs: evidence-based security design for the fund #11
• feat: expose the program idl as a flake output #22 ← you are here
• docs: ADR for permissionless NAV attestation #23
• docs: link security-design.md and adrs/ from the README, correct NAV wording #26

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Motivation

moneymentum needs the program's Anchor IDL to generate client bindings, and a
git submodule was the only way to reach it. Submodules bring gitlink/checkout
friction everywhere they touch (CI, worktrees, fresh clones); all the consumer
actually needs is one build artifact.

Solution

Expose the IDL as a flake output: packages.idl (also the default package)
builds idl/fund.json, so moneymentum can take fund as a regular flake input
and drop the submodule. The pinned revision moves from a gitlink to the
consumer's flake.lock.

The derivation runs anchor idl build with the host toolchain only -- no
cargo-build-sbf, no platform-tools -- so unlike the SBF build it works on
every system, not just aarch64-darwin. Two PATH shims (a cargo wrapper that
strips the rustup-style +toolchain argument and a no-op rustup) bridge
anchor's rustup assumptions to the nix toolchain, and the underlying
cargo test is restricted to --lib because the litesvm integration tests
include_bytes! the SBF artifact this derivation deliberately does not build.

Testing

• nix build .#idl produces a complete IDL (program address, create_fund +
  deposit instructions, full error enum) -- richer than the stale
  target/idl/fund.json from the last manual anchor build.
• The derivation is also wired into checks, so nix flake check builds it.

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