fix(build): track bin/dune (executable definition) — fix gitignore negation

CI's `Run codegen WASM tests` step was failing with `Program 'affinescript'
not found!` because the test runner couldn't locate the compiled binary.
The cause: `bin/dune` was never tracked.

Root cause was a gitignore semantics issue. The .gitignore had:

    /bin/
    !bin/*.ml
    !bin/dune

`/bin/` excludes the directory itself. Per git docs, "It is not possible
to re-include a file if a parent directory of that file is excluded" —
so `!bin/*.ml` and `!bin/dune` were silent no-ops. `bin/main.ml` happened
to be tracked from before the gitignore rule was added, but `bin/dune`
was never tracked, so origin's `bin/` had only main.ml and dune saw no
executable to build.

Fix: change `/bin/` to `/bin/*` (excludes contents, not the directory)
so the negations actually re-include their targets. Also force-added
`bin/dune` (the executable definition: `(executable (name main)
(public_name affinescript) (package affinescript) (libraries
affinescript cmdliner fmt fmt.tty))`) so origin gets it for the first
time.

After this lands, `dune build` produces `_build/default/bin/main.exe`
and the codegen test runner's first compiler-detection branch hits.

Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath

.gitignore

@@ -51,7 +51,12 @@ __pycache__/
 # Ada/SPARK
 *.ali
 /obj/
-/bin/
+# `/bin/` excludes the directory itself, which makes git unable to re-include
+# files inside it (negations can't re-include children of an excluded dir).
+# `/bin/*` excludes the *contents* so `!bin/*.ml` and `!bin/dune` actually
+# re-include their targets. The previous `/bin/` form silently dropped
+# bin/dune from origin and broke CI's executable build.
+/bin/*
 !bin/*.ml
 !bin/dune
 

bin/dune

@@ -0,0 +1,7 @@
+; SPDX-License-Identifier: MIT OR AGPL-3.0-or-later
+
+(executable
+ (name main)
+ (public_name affinescript)
+ (package affinescript)
+ (libraries affinescript cmdliner fmt fmt.tty))