chore(dune-project): restore symlink to .build/dune-project (#307)

The relicense sweep (#304) used `sed -i` which atomic-rename-replaces
the target file, breaking the symlink: `dune-project` flipped from
mode 120000 (symlink) to 100644 (regular file).  Both files have been
content-identical since, so nothing's broken — but the symlink was
the original repo layout (presumably so dune's per-package
`(version …)` + opam regeneration lives under `.build/` alongside
other build-output staging).

Restore it: `dune-project` is once again a symlink to
`.build/dune-project`.  Verified post-restore: `dune build --release`
clean, binary `--version` reports `0.1.1`.  Single behavioural change
is that an edit to either path now propagates to the other again, as
originally intended.

If a future bulk sed across the repo encounters this symlink again,
the safe pattern is to operate on `git ls-files` (which lists each
symlink only once via its declared path) — or to edit `.build/dune-project`
directly, which keeps the symlink intact.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

dune-project

@@ -0,0 +1 @@
+.build/dune-project