The four libraries that use cppo (compiler/{common,ml,core,ext}) had a
library-wide `(preprocess (action (run cppo ...)))` stanza, which dune
disallows in combination with `(instrumentation ...)`. That blocked
adding bisect_ppx coverage on the modules where most of the type
checker and error reporting lives.

Of the ~250 .ml files across these four libraries, only 10 actually
use cppo directives — all toggling the existing BROWSER and RELEASE
flags for the JSOO playground build and stripped debug logging. The
fix is mechanical:

- Rename those 10 source files to `*.cppo.ml` (or `*.cppo.mli`)
- Add a per-file `(rule (target X.ml) (deps X.cppo.ml) (action cppo))`
  for each, matching the convention compiler/ext already uses for its
  template files (hash_set.cppo.ml, vec.cppo.ml, etc.)
- Drop the library-wide `(preprocess ...)` stanzas

The same `%{env:CPPO_FLAGS=}` substitution is preserved in each rule,
so the dev / release / static / browser profile behavior in
`compiler/dune` is unchanged. js_reserved_map needs cppo's `-V OCAML:`
flag for its `#if OCAML_VERSION >= (5, 0, 0)` directive, which is
preserved on its rule specifically.

Side benefit: cppo only runs on the 10 files that need it instead of
every file in those libraries.

The flags I'd written for the report subcommands didn't match what
bisect_ppx 2.8 actually accepts:

- `--ignore-files <regex>` (used for excluding generated files) does
  not exist. The 2.8 API takes path globs via `--expect`/
  `--do-not-expect`. Dropped the regex filter for now — generated
  files don't add meaningful noise to the report. We can revisit
  excluding `compiler/ext/*_string.ml` etc. with the path-based API
  later if it becomes useful.
- `--ignore-missing-files` is only accepted by `html` and `cobertura`,
  not by `summary`. Removed it from summary invocations.
- `cobertura` takes the output file as a positional argument, not via
  `-o`. Fixed.

Also pulls in the regenerated rescript.opam (`bisect_ppx` was added as
a `with-dev-setup` dep in rescript.opam.template earlier; dune
regenerates the .opam file on build).

`make coverage-super-errors` now runs end-to-end and produces a
per-file table for the type-checker / error-reporting modules.

ocamlformat was failing on the renamed .cppo.ml/.mli files because the
ignore list still referenced their old paths. Replaced the explicit
file list with `**/*.cppo.ml` and `**/*.cppo.mli` globs, which covers
the renamed files plus the existing `compiler/ext/*.cppo.ml` templates
that were already listed individually. Future cppo files added under
the new convention are covered automatically.

Also includes the auto-promoted dune format applied to the
js_reserved_map rule in compiler/ext/dune (`dune build @fmt`
reformatted the multi-arg cppo invocation onto one arg per line).

Trim the coverage surface to what's actually needed for local
inspection + agent queries:

- Remove `coverage-report-cobertura` — Cobertura XML is only useful
  for CI dashboards (Codecov/Coveralls upload). Not wiring CI for now.
- Remove `coverage-super-errors` and `coverage-run-super-errors` —
  super_errors fixtures are already executed under `make coverage`
  via scripts/test.js -all (which iterates tests/build_tests/*).
  Querying for super_errors-relevant modules is easy with jq on
  coverage.json or a grep on `summary --per-file`.
- Add `coveralls` JSON output to `coverage-report`. Goes to
  _coverage/coverage.json with the structure documented in the
  Makefile header so an agent / jq one-liner can consume it directly.
- Inline the COVERAGE_IGNORE variable since `--ignore-missing-files`
  is the only flag now.

User-facing surface: `make coverage`, `make clean-coverage`, plus
internal phases (coverage-build, coverage-prepare, coverage-lib,
coverage-run, coverage-report) for re-running individual stages.

Runs make coverage on a single Linux/ARM job and uploads the
coveralls-format report to Codecov. The codecov.yml keeps checks
informational and suppresses inline annotations so PR coverage
feedback stays low-noise for contributors.

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

coverage-build copies bisect_ppx-instrumented binaries into BIN_DIR;
without cleanup, subsequent make/make test runs silently reuse them.
Remove the instrumented executables and compiler build stamp at the
end of make coverage so the next build rebuilds uninstrumented.