Traverse used JS implementation which is OK except that the array indexing was starting from 0.

Fix `traverse` failing at runtime

Follows the merged traverse fix (#2 by @Renegatto). On top of it:

- Fix mapWithIndexArray: it passed the 1-based Lua loop index to f, but
  PureScript mapWithIndex is 0-based, so f received i instead of i-1
  (originally reported in #1 by @sarahnya).
- Add a focused regression suite (test-regression/) run by scripts/test:
  traverse over Maybe is length/order-preserving at every size (>3 hits
  the pivot/append branch, which needs the Array Semigroup fix shipped in
  prelude v7.2.2), traverse over the Array applicative, and 0-based
  mapWithIndex.
- Align tooling: purescript-overlay dev shell on Lua 5.1 (drops easy-ps /
  insecure nodejs), canonical nix CI replacing the bower workflow,
  fail-fast scripts, and bump the package set to psc-0.15.15-20260614
  (prelude v7.2.2). Supersedes the earlier normalization PR #3.

Closes #1

chore: finalize traverse fix — mapWithIndex, regression test, tooling alignment

CI: drop accept-flake-config (supply-chain risk; caches already pinned),
run luacheck with --std lua51 --no-unused-args (matches the Lua 5.1 target
and the curried-FFI idiom), invoke the test step via bash so it no longer
needs the execute bit.

AGENTS.md is the single instruction file the coding agents read natively;
CLAUDE.md is a one-line @AGENTS.md import so Claude Code shares it. Also
removes dead JavaScript-era package files. Tooling and docs only, no src/.

chore: align CI to the hardened canon, add AGENTS.md

Repos moved from Unisay/* to the purescript-lua org. Update the pslua
flake input and lockfile owner (same rev 94c13ce), the packages.dhall
upstream-lua release URL, and the AGENTS.md links, so nothing relies on
the old-owner redirect.

chore: point pslua + package-set links at purescript-lua org

Wire treefmt via treefmt-nix: nixfmt, dhall format, purs-tidy (.tidyrc.json)
and LuaFormatter for the FFI (.lua-format, kept over StyLua because it
preserves the parentheses pslua's parser needs). `nix fmt` formats; the dev
shell installs a content-based pre-commit hook and CI runs
`nix fmt && git diff --exit-code` (content-based, since the in-place
formatters bump mtime and would trip treefmt --fail-on-change). Lua lines
budget 130 cols, matching the raised `luacheck --max-line-length`. The bulk
of the diff is the first format pass.

chore: add treefmt formatting (nix fmt + pre-commit + CI check)

Replace the dev-shell installer that wrote .git/hooks/pre-commit with a
tracked .githooks/pre-commit wired via `git config core.hooksPath .githooks`.

Fixes three issues with the old hook: it gated on `[ -d .git ]` (false in
worktrees/submodules where .git is a file), it clobbered any existing
.git/hooks/pre-commit on every `nix develop`, and `nix fmt … || exit 0`
swallowed real formatter failures. The new hook skips only when `nix` is
absent (CI is the authoritative gate) and otherwise blocks on a `nix fmt`
failure or reformat.

Toolchain consistency: pin the pslua dev flake input to the current
compiler main across the ecosystem. flake.lock only; the package set
consumes sources + FFI, not this dev input, so no re-tag is needed.

Build against the published purescript-lua package set via
workspace.packageSet.url instead of the old spago.dhall + packages.dhall merge.
The new spago drives spago.yaml/spago.lock and dropped Dhall.

- spago.dhall/packages.dhall -> spago.yaml (package name lua-foldable-traversable;
  the new spago rejects the purescript- prefix). backend.cmd "true" keeps codegen
  on corefn; scripts/build runs pslua over it.
- flake.nix: spago 0.21.0 -> 1.x; drop the now-unused dhall dev-shell tool.
- treefmt.nix: drop the dead Dhall formatter. AGENTS.md: spago 1.x, src/packages.json.
- CI action versions bumped to current majors.
- Port the regression test off the Dhall harness: spago-test.dhall (which used
  spago's dropped --config/Dhall) becomes a package.test section; the test source
  moves test-regression/Main.purs -> test/Main.purs (new spago globs test/), and
  scripts/test now builds, runs pslua --entry Main, then lua. Remove the vestigial
  upstream test/Main.purs + test/Main.js (never built under the old sources=src).

Dead config: nix fmt formats via treefmt.nix, and nothing reads treefmt.toml
(the pre-commit hook and CI both run nix fmt).

Adds a CHANGELOG.md managed with scriv (fragments in changelog.d/), provides
scriv in the flake dev shell, and records the fork's Lua 5.1 FFI release line.
Where the upstream PureScript changelog existed it is preserved below the fork
section. The AGENTS.md Releasing note now points at the scriv flow instead of
"no changelog entry".

Part of the ecosystem-wide changelog adoption
(purescript-lua/purescript-lua#101); canon in package-set ADR 0009.