fix(docs): make the documented bootstrap work on a fresh clone

bin/ is gitignored and not checked in, so the first command in README.md,
COMPILING.md, CONTRIBUTING.md, tools/x/main.ludic and on the site —

    clang selfhost/ludicc.seed.ll -o bin/ludicc && ...

failed on a clean checkout with `ld: open() failed, errno=2 for 'bin/ludicc'`.
Verified against a fresh clone. Every copy now leads with `mkdir -p bin`, which
is what the CI workflows had been doing all along.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

COMPILING.md

@@ -17,7 +17,7 @@
 >
 > ```bash
 > # one-time bootstrap: clang assembles the seed, then ludicc compiles bin/x
-> clang selfhost/ludicc.seed.ll -o bin/ludicc && bin/ludicc tools/x/main.ludic -o bin/x
+> mkdir -p bin && clang selfhost/ludicc.seed.ll -o bin/ludicc && bin/ludicc tools/x/main.ludic -o bin/x
 > bin/x build                             # rebuild the whole toolchain into bin/
 >                                         #   (ludicc, ludic, x, ludic-fmt, ludic-lsp)
 > bin/ludicc examples/games/snake.ludic -o bin/snake   # compile

README.md

@@ -39,7 +39,7 @@ Bootstrap it once from a clean checkout (the only step Ludic can't do for
 itself, since compiling Ludic needs a compiler) with clang alone:
 
 ```bash
-clang selfhost/ludicc.seed.ll -o bin/ludicc && bin/ludicc tools/x/main.ludic -o bin/x
+mkdir -p bin && clang selfhost/ludicc.seed.ll -o bin/ludicc && bin/ludicc tools/x/main.ludic -o bin/x
 ```
 
 Then build the whole toolchain and run a game:

changes/bootstrap-mkdir-bin.md

@@ -0,0 +1,7 @@
+bump: patch
+type: fix
+The documented bootstrap works on a fresh clone. `bin/` is gitignored and not
+checked in, so `clang selfhost/ludicc.seed.ll -o bin/ludicc` — the first command
+in the README, in COMPILING, in CONTRIBUTING and on the site — failed with
+`ld: open() failed, errno=2 for 'bin/ludicc'`. Every copy now begins with
+`mkdir -p bin`, which is what CI had been doing all along.