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.