export fn and finch build --shared: libraries a C program loads (game scripts)

export fn name(...) is a C function by that name (numbers, bool, char, pointers in and out).
--shared builds lib<name>.so / <name>.dll with no main. finch_module_on_fatal(handler) lets the
host take runtime errors over (longjmp out) instead of the process exiting.
tests/shared.sh: a C host loads one, calls it and survives an index error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -34,6 +34,8 @@ jobs:
           build/finch version
       - name: Tests
         run: tests/run.sh
+      - name: Shared library
+        run: tests/shared.sh
       - name: Tests under valgrind
         if: matrix.arch == 'x64'
         run: MEMCHECK=1 tests/run.sh