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>

.forgejo/workflows/bootstrap.yml

@@ -0,0 +1,50 @@
+name: bootstrap
+
+# The headline guarantee of a self-hosted, C-free toolchain: assembled from the
+# checked-in IR seed with clang alone, the compiler recompiles its own source
+# and reproduces that seed **byte-for-byte**. A regression here (a compiler
+# change that was never reseeded) is otherwise invisible until someone runs it
+# locally. This job fails the moment out.ll != seed.ll.
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch: {}
+
+jobs:
+  cfree-fixpoint:
+    runs-on: docker
+    container: node:20-bookworm
+    steps:
+      - name: Install clang-16
+        run: |
+          set -eu
+          export DEBIAN_FRONTEND=noninteractive
+          apt-get update -qq
+          apt-get install -y -qq --no-install-recommends clang-16 git ca-certificates
+          clang-16 --version | head -1
+
+      - name: Check out the triggering commit
+        run: |
+          set -eu
+          git config --global --add safe.directory '*'
+          git clone https://git.workshopsoft.io/workshopsoft/ludic.git .
+          git checkout "${GITHUB_SHA}" 2>/dev/null || git checkout "${GITHUB_REF_NAME:-main}"
+          git log --oneline -1
+          # See ci.yml for why the Linux build injects the stdio shim via LUDIC_CC.
+          echo "LUDIC_CC=clang-16 $(pwd)/tools/ci/linux_stdio_shim.ll" >> "$GITHUB_ENV"
+          echo "LUDIC_HOME=$(pwd)" >> "$GITHUB_ENV"
+
+      - name: Bootstrap x from the seed
+        run: |
+          set -eu
+          mkdir -p bin
+          clang-16 tools/ci/linux_stdio_shim.ll selfhost/ludicc.seed.ll -o bin/ludicc
+          bin/ludicc tools/x/main.ludic -o bin/x
+
+      - name: Rebuild the compiler from the seed and assert byte-identity
+        # `x bootstrap-cfree` assembles the seed with clang, has that seed
+        # compiler recompile selfhost.ludic to out.ll, and `cmp`s out.ll against
+        # the checked-in seed. It returns non-zero if they differ — i.e. if the
+        # seed is stale relative to the compiler source.
+        run: bin/x bootstrap-cfree