chore(nim-bindings): replace dynlib dlopen with plain importc (#61)

* chore(nim-bindings): replace dynlib dlopen with plain importc

The dynlib pragma hard-coded a library path and resolved it via dlopen() at
runtime, preventing static linking and forcing a specific load-time path.
Using bare {.importc.} lets consumers choose: link liblibchat dynamically
at link time (--passL:-llibchat) or link it statically into their binary.

* Rust -> Nim ABI  (#62)

* Use correct build hook

* force sret like return from rust code for nim compatibility

* Fix target mismatch

* Update usages

* ci: add nim-bindings-test

* fix(nim-bindings): fix ABI mismatch in destroy_* FFI functions and add defer-based cleanup

Nim's C backend silently transforms large struct parameters (>16 bytes) into
pointer parameters when calling importc functions. The destroy_* functions were
declared taking T by value in Rust, but Nim always passed &T — causing Rust to
read garbage from the stack on x86-64 (SIGILL on CI) while accidentally working
on ARM64 macOS due to that ABI coincidentally also using pointers for large structs.

Fix by changing all destroy_* functions to take &mut T and using drop_in_place,
which is the correct idiom for dropping a value through a pointer.

On the Nim side, replace scattered manual destroy calls with defer, which
guarantees cleanup on all exit paths and prevents use-after-destroy bugs.

---------

Co-authored-by: Jazz Turner-Baggs <473256+jazzz@users.noreply.github.com>

.github/workflows/ci.yml

@@ -2,11 +2,11 @@ name: CI
 
 on:
   push:
-    branches: [ main ]
+    branches: [main]
   pull_request:
-    branches: [ main ]
+    branches: [main]
 
-env: 
+env:
   CARGO_TERM_COLOR: always
 
 jobs:
@@ -36,3 +36,21 @@ jobs:
       - run: rustup update stable && rustup default stable
       - run: rustup component add rustfmt
       - run: cargo fmt --all -- --check
+
+  nim-bindings-test:
+    name: Nim Bindings Test
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustup update stable && rustup default stable
+      - name: Install Nim via choosenim
+        run: |
+          curl https://nim-lang.org/choosenim/init.sh -sSf | sh -s -- -y
+          echo "$HOME/.nimble/bin" >> $GITHUB_PATH
+      - run: nimble install -dy
+        working-directory: nim-bindings
+      - run: nimble pingpong
+        working-directory: nim-bindings