ci: add build + test + bootstrap-cfree workflows for the Forgejo runner

Until now the only workflow was docs.yml — nothing gated a change on the
compiler even building, on `x test` / `x test-tools`, or on the headline C-free
self-rebuild reproducing the seed. Add two Forgejo Actions workflows on the same
`docker` runner the docs job uses.

The toolchain is macOS-first: the self-hosted compiler emits the Darwin libc
standard-stream globals (`__stdoutp`/`__stderrp`), the one thing that stops its
IR from linking on Linux. Everything else is portable — clang-16 assembles the
seed cleanly and the C-free bootstrap reproduces it byte-for-byte on Linux too.
So rather than require a macOS runner (none is registered), bridge that single
gap with a tiny **C-free LLVM-IR shim** (tools/ci/linux_stdio_shim.ll) that
defines the Darwin-named globals over glibc's stdout/stderr, injected into every
clang link via LUDIC_CC. The language keeps its no-C-compiler guarantee.

Workflows:
- ci.yml — bootstrap the toolchain from the seed, then `x test` + `x test-tools`
  + the docs-cover-the-implementation checks, on push to main and PRs.
- bootstrap.yml — `x bootstrap-cfree`: assert the seed rebuilds itself
  byte-for-byte (returns non-zero on drift).

Make the suites host-aware so a Linux run is green without hiding anything: a
new is_darwin()/skip() pair (tools/x/prelude.ludic) makes the cases that are
genuinely macOS-ABI bound — the Cocoa-windowed `ludicc -o` link, the golden
render hashes (blessed on macOS; text raster differs by a hair elsewhere), the
Os known-folder/uname surface, Fs.list and the LSP workspace walk (both read the
BSD dirent layout) — print a visible `skip` off Darwin instead of failing. On
macOS every one of them still runs: suites stay 56 / 29 / 29 green there, and
run 51 / 28 (+skips) on Linux, bootstrap-cfree byte-identical on both.

The formatting gate is ludic-fmt *idempotence* (already in `x test-tools`), not
`fmt(x) == x`: this codebase deliberately preserves hand alignment, so a strict
"already formatted" check would fight that contract.

A prebuilt CI image with clang-16 + python3 baked in is the obvious follow-up
speed-up (ties into the packaging work in #33).

Closes #32

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/x/selfhost_test.ludic

@@ -25,6 +25,11 @@ function game_case(path: pointer, keys: pointer) -> void {
   if not game_build_ok("bin/ludicc", `examples/{path}.ludic`, `/tmp/x_g_{nm}`) { bad(`{path}: build`); return }
   run("mkdir -p build")
   run(`printf '%s' '{keys}' | /tmp/x_g_{nm} >/dev/null 2>&1`)
+  # The build + headless run above guard against a compile or runtime break on
+  # every host. The pixel-exact golden comparison, though, is against a render
+  # blessed on macOS; text rasterization differs by a hair on other hosts, so
+  # off Darwin we stop at "it built and ran" rather than fail on the hash.
+  if not is_darwin() { skip(`{path} render (golden hash blessed on macOS)`); return }
   let got = capture_line("shasum -a 256 build/out.ppm | cut -d' ' -f1")
   let want = capture_line(`grep '^{path} ' selfhost/golden/renders.sha256 | cut -d' ' -f2`)
   if (want == "") { bad2(path, "no golden hash (run: bin/x golden)"); return }