[build] drop the whole Scala Native work directory in CI, not just generated/

[DamianReeves]

Fixes #1821.

What was happening

The Native rows failed at link with a run of

clang: error: no such file or directory: .../generated/<hash>.ll.o

and no diagnostic from any earlier phase. Deterministic: the same nine hashes on both arches, shared across kyo-aeron (#1818) and kyo-actor (#1809), because those are the compilation units whose NIR nothing in the build perturbs.

The issue guessed that clang was failing and its stderr was being discarded. It is not: clang is never started for those units.

Cause

Test / nativeLink deleted <module>/native/target/scala-*/native-test/generated after each link, to keep ~40 modules inside the runner's disk. It left package2hash, Scala Native's incremental-codegen state naming those IR files, as a sibling of the directory it deleted.

Scala Native's build-skip checksum covers classpath mtimes, so a module whose dependency the test phase recompiled relinks rather than short-circuiting. Then:

1. CodeGen.separateIncrementally reads package2hash, sees the unit unchanged, and returns the .ll path without checking the file exists. Its only guard is assert(ownerDirectory.toFile.exists()) on the parent directory, which emit recreated a few lines earlier.
2. LLVM.needsCompiling compares two absent files, 0 > 0, skips clang, and returns the object path anyway.
3. Both phases log success. The link is the first thing to touch the disk.

Fix

Prune the whole work directory except build-checksum. That file plus the linked binary (one level up, outside the work directory) is the entire input to Scala Native's "Build skipped" check, so a repeat link still short-circuits. Dropping the rest wholesale is what keeps the directory self-consistent, and it reclaims more disk than before.

Guard

scripts/native-relink-selftest.sh: link, assert the work directory holds only build-checksum, bump one classpath mtime so the checksum misses without recompiling anything, relink, assert it succeeds and referenced no object codegen never wrote. --self-test covers the harness against a faked sbt, negative control included. The Native rows run it before the suite, on kyo-data, and reuse the binary it links.

Verification

Reproduced locally on macOS with kyo-dataNative, hitting the same nine hashes as CI.

Check	Result
native-relink-selftest.sh --self-test	7/7
guard against the fixed build	pass
guard against the old hook	fails on work directory not pruned
old hook plus a checksum miss	exit 1, nine missing .ll.o
no-change relink after the prune	Build skipped, fast path intact
actionlint, shellcheck, scalafmtSbt	clean

Upstream

The underlying defect is in Scala Native 0.5.12 and reproduces without kyo: https://github.com/DamianReeves/scala-native-bug-reporting

Not yet filed against scala-native.