a94559f4

crate2nix (IFD) experiment

A comparison experiment to the crane-based `your-first-flake` branch: builds
bombadil with **crate2nix in IFD mode** (`generatedCargoNix`) — **per-crate
derivations, no committed `Cargo.nix`**.

## What it shows
- **Finest-grained caching.** One derivation per crate: editing one workspace
  crate rebuilds only that crate and its dependents, never the ~377 external
  dependency crates.
- **The cost for this repo.** It needs a patch to crate2nix itself
  (`nix/crate2nix-git-workspace.patch`) so that `boa_engine` — a git dependency
  that is a member of the boa monorepo's workspace — vendors with a
  self-contained, de-inherited manifest. Without it, generation fails with
  `failed to find a workspace root`. Plus a `bombadil-cli` build-script override
  for the workspace-relative inspect path, and IFD at eval time.

## crane vs crate2nix

| | crane (`your-first-flake`) | crate2nix (this branch) |
| --- | --- | --- |
| Dependency caching | one shared derivation; source edits never rebuild it | one derivation **per crate** |
| Committed generated file | none | none (IFD) |
| IFD | no | **yes** |
| git workspace-member deps (boa_engine) | handled natively by cargo | needs the patch here |
| Custom build.rs (ghostty, inspect) | handled in the build derivation | needs per-crate overrides |

For bombadil, crane is the lower-maintenance choice; this branch shows the
trade-off concretely.

## Verify per-crate caching yourself

Records the external dependency-crate derivations in the build closure of the
default package, edits a real workspace `.rs` file, and asserts they are all
byte-for-byte identical afterwards (only the edited crate and its dependents
change). Reverts the edit afterwards.

```bash
#!/usr/bin/env bash
set -euo pipefail
cd "$(git rev-parse --show-toplevel)"

ATTR='.#default'
VICTIM='lib/bombadil/src/lib.rs'

external_crate_drvs() {
  nix-store --query --requisites "$(nix eval --raw "${ATTR}.drvPath")" \
    | grep -E '\.drv$' | grep -E -- '-rust_' \
    | grep -vE -- '-rust_(bombadil|small-string)' | sort
}

before="$(external_crate_drvs)"
echo "external dependency-crate derivations: $(echo "${before}" | wc -l)"

cleanup() { git checkout -- "${VICTIM}" 2>/dev/null || true; }
trap cleanup EXIT

printf '\n// experiment: rebuilds only this crate, not the deps.\npub fn _marker() {}\n' >>"${VICTIM}"
git --no-pager diff --stat -- "${VICTIM}"

after="$(external_crate_drvs)"
[ "${before}" = "${after}" ] && echo "PASS — no external dependency-crate rebuilds." || { echo "FAIL"; diff <(echo "${before}") <(echo "${after}"); exit 1; }
```

## CI
Passing on NixCI (production):
https://nix-ci.com/gh:NorfairKing:bombadil/crate2nix-experiment