Enable bisect_ppx coverage instrumentation via per-module preprocessing

Solved the preprocessor conflict by using per-module dune stanza:
- CCAtomic, CCList, CCVector: Use cpp.exe (need version conditionals)
- All other core modules: Use bisect_ppx for coverage
- containers_cbor: Full bisect_ppx coverage (no cpp needed)

This allows coverage collection on ~95% of the codebase while
maintaining version compatibility preprocessing where needed.

Initial coverage results:
- CBOR: 87.50% (203/232 points) from RFC test vectors
- Core: instrumented except 3 modules

To generate coverage:
  BISECT_FILE=_coverage/bisect dune runtest
  bisect-ppx-report html -o _coverage/html

src/cbor/dune

@@ -1,7 +1,5 @@
 (library
  (name containers_cbor)
  (libraries containers)
- (preprocess
-  (action
-   (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
+ (preprocess (pps bisect_ppx))
  (public_name containers.cbor))

src/core/dune

@@ -3,8 +3,10 @@
  (public_name containers)
  (wrapped false)
  (preprocess
-  (action
-   (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
+  (per_module
+   ((action (run %{project_root}/src/core/cpp/cpp.exe %{input-file}))
+    CCAtomic CCList CCVector)
+   ((pps bisect_ppx))))
  (flags :standard -nolabels -open CCMonomorphic)
  (libraries either containers.monomorphic containers.domain))