Scaffolding. Move purescript-lua-arrays onto the package-set canon: an
overlay flake (purs 0.15.16, spago 0.21.0, Lua 5.1, pslua at the current
main), hardened CI, scripts/build, AGENTS.md + CLAUDE.md, a clean
.gitignore, and the purescript-lua org package-set URL. Drop the
.eslintrc, bower.json and package.json.

FFI fix. The foreign modules used Lua 5.2/5.3 builtins absent in 5.1:
src/Data/Array.lua called table.pack(table.unpack(...)) in unconsImpl,
_insertAt, _deleteAt, _updateAt and the sort, now { unpack(...) }; and
src/Data/Array/ST.lua used table.move in freeze/thaw, pushAll, unshiftAll,
splice and sortBy, now a hand-written overlap-safe move with the same
semantics as Lua 5.3 table.move (it copies backwards when the source and
destination overlap to the right).

Tests. scripts/test runs test/regression/array_st.lua, a lean guard over
exactly the rewritten paths: freeze independence, pushAll, splice in both
overlap directions, a stable sort, and the unpack-based uncons/insertAt/
deleteAt/updateAt. Verified locally with build, the regression run, and
luacheck --std lua51 all green.

feat: bring arrays to the canon and make the FFI Lua 5.1-safe

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.

Three Lua FFI bugs, each now guarded by a failing-then-passing regression
test (test/regression/{array_st,nonempty}.lua):

- unshiftAllImpl prepended via move() over the front slots, overwriting
  existing elements instead of shifting them right. Now shifts then copies.
  Fixes purescript-lua/purescript-lua#74.
- spliceImpl only shifted the tail: it never inserted bs, never shrank on
  remove>insert, and returned the whole array instead of the removed slice.
  Now implements full JS splice semantics. Fixes purescript-lua/purescript-lua#75.
- traverse1Impl's Cont/ConsCell were ported as 'this'-mutating procedures
  but called with data args only, erroring on first use. Now they return
  fresh tables. Fixes purescript-lua/purescript-lua#73.

The pre-existing splice tests asserted the buggy move-only output; they are
replaced with correct-semantics cases. Verified under Lua 5.1.

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

- spago.dhall/packages.dhall -> spago.yaml (package name lua-arrays; the new
  spago rejects the purescript- prefix). backend.cmd "true" keeps codegen on
  corefn; scripts/build runs pslua over it as before.
- 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 (checkout v5, install-nix v31).
- Remove the upstream PureScript test sources under test/Test (never built: old
  sources globbed src/ only, and they carry JS FFI that doesn't target Lua). The
  Lua regression tests under test/regression stay.

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

Every other set fork carries this section; arrays was the only one
missing it, so the ADR canon was not discoverable from here.

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.