Build system: a failed first build forces full rebuilds, and clean breaks a running watch #9241

Two ways the build system loses or misreads its state in lib/. Both reproduce with rescript 13.0.0-alpha.8 (found while reviewing #9238).

1. After a failed first build, every build cleans everything

# src/A.res: let x: int = "s"     src/B.res: let y = 1
rescript build   # Cleaned 0/0 … exit 1
rescript build   # Cleaned previous build due to compiler update / Cleaned 0/1 / Compiled 2 modules, exit 1
rescript build   # same again

Every build after the first failed one wipes lib/bs and lib/ocaml and recompiles all modules (B as well), with a misleading "compiler update" message, until a build succeeds.

compiler_info.ml:133-136 (needs_clean) treats an existing lib/ocaml/.compiler.log without compiler-info.json as a stale build, but compiler-info.json is only written after a successful build (build.ml:452-457).

2. rescript clean while rescript watch runs breaks the watcher

# src/A.res: let x = 1     src/B.res: let y = A.x
rescript watch &
rescript clean     # Cleaning x, exit 0
# edit src/A.res

Every later rebuild fails with Sys_error("<root>/lib/ocaml/A.ast: No such file or directory"), and the watcher never recovers.

clean.ml:83 only takes the build lock, which the watcher holds only while it builds. clean should refuse while lib/watch.lock is held (or the watcher should rebuild from scratch when its outputs disappear). The raw Sys_error is printed by build.ml:556-558.