Use instrumentation stanza for CBOR (cleaner approach)

Changed CBOR from 'pps bisect_ppx' to 'instrumentation (backend bisect_ppx)'
which is dune's recommended approach for coverage when not using custom
preprocessing.

Benefits:
- Cleaner dune syntax
- Use --instrument-with bisect_ppx flag (no env vars needed)
- Coverage files auto-managed in _build
- More idiomatic dune

Updated documentation to reflect new usage:
  dune runtest --instrument-with bisect_ppx
  bisect-ppx-report summary --coverage-path=_build

Core library still uses 'pps bisect_ppx' due to per-module preprocessing
requirements (cpp.exe for 3 modules).

COVERAGE_SETUP.md

@@ -21,11 +21,14 @@ The ocaml-containers project uses a custom preprocessor (`cpp.exe`) for OCaml ve
 ### CBOR Library (`src/cbor/dune`)
 
 ```ocaml
-(preprocess (pps bisect_ppx))
+(instrumentation (backend bisect_ppx))
 ```
 
 **Result:** Full coverage instrumentation (100%)
 
+**Note:** CBOR uses `instrumentation` stanza (cleaner) while core uses `pps` 
+(required for per-module preprocessing compatibility)
+
 ## Modules Excluded from Coverage
 
 Only 3 modules require cpp preprocessing:
@@ -40,24 +43,26 @@ These modules use `[@@@ifge 4.8]` style conditionals for OCaml version compatibi
 ### Generate Coverage Data
 
 ```bash
-# Run tests with coverage
-BISECT_FILE=_coverage/bisect dune runtest
+# Run tests with coverage using instrumentation
+dune runtest --instrument-with bisect_ppx
 
 # Or run specific test suite
-BISECT_FILE=_coverage/bisect dune runtest tests/cbor
+dune runtest tests/cbor --instrument-with bisect_ppx
+
+# Coverage files are written to _build/default/tests/*/*.coverage
 ```
 
 ### Generate Reports
 
 ```bash
 # Summary
-bisect-ppx-report summary --coverage-path=_coverage
+bisect-ppx-report summary --coverage-path=_build
 
 # Per-file breakdown
-bisect-ppx-report summary --coverage-path=_coverage --per-file
+bisect-ppx-report summary --coverage-path=_build --per-file
 
 # HTML report
-bisect-ppx-report html --coverage-path=_coverage -o _coverage/html
+bisect-ppx-report html --coverage-path=_build -o _coverage/html
 ```
 
 ### View HTML Report
@@ -122,20 +127,20 @@ bisect-ppx-report html --coverage-path=_coverage -o _coverage/html
 ### Example Workflow
 ```bash
 # Initial run
-BISECT_FILE=_coverage/bisect dune runtest tests/cbor
-bisect-ppx-report summary --coverage-path=_coverage --per-file
+dune runtest tests/cbor --instrument-with bisect_ppx
+bisect-ppx-report summary --coverage-path=_build --per-file
 
 # View gaps
-bisect-ppx-report html --coverage-path=_coverage -o _coverage/html
+bisect-ppx-report html --coverage-path=_build -o _coverage/html
 firefox _coverage/html/src/cbor/containers_cbor.ml.html
 
 # Add tests to cover gaps
 # ... edit tests/core/t_cbor.ml ...
 
-# Re-run
-rm _coverage/*.coverage
-BISECT_FILE=_coverage/bisect dune runtest tests/cbor
-bisect-ppx-report summary --coverage-path=_coverage
+# Re-run (dune automatically regenerates coverage)
+dune clean
+dune runtest tests/cbor --instrument-with bisect_ppx
+bisect-ppx-report summary --coverage-path=_build
 ```
 
 ## Benefits Achieved
@@ -156,9 +161,13 @@ The `instrumentation` stanza cannot be combined with `(preprocess (action ...))`
 
 via per-module configuration.
 
-### Why not `--conditional`?
+### Why mix `instrumentation` and `pps bisect_ppx`?
 
-We tried `(pps bisect_ppx --conditional)` initially, but it requires `BISECT_ENABLE=yes` to be set, which is less ergonomic. Without `--conditional`, coverage is always collected (small performance overhead but simpler workflow).
+- **CBOR:** Uses `instrumentation` stanza (cleaner, dune's recommended approach)
+- **Core:** Uses `pps bisect_ppx` in per-module preprocessing (works with action preprocessing)
+
+The `instrumentation` stanza is preferred but cannot be used with `(preprocess (action ...))`. 
+We use it where possible (CBOR) and fall back to `pps` where needed (core).
 
 ### Alternative Approaches Considered
 

src/cbor/dune

@@ -1,5 +1,5 @@
 (library
  (name containers_cbor)
  (libraries containers)
- (preprocess (pps bisect_ppx))
+ (instrumentation (backend bisect_ppx))
  (public_name containers.cbor))