CI: flip Idris2 ABI Proofs to strict (remove continue-on-error)

[hyperpolymath]

Follow-up to #29 / #31 (Wave-3 Chapel rehabilitation).

The check-abi job in .github/workflows/hpc-ci.yml currently has continue-on-error: true with comment "Idris2 CI setup can be fragile". Empirically: on the rerun of #31, this job failed with idris2: command not found after the Idris2 source-bootstrap step. The source-bootstrap is the fragile bit, not docudactyl's proofs.

To flip the job to strict:

1. Replace the make bootstrap SCHEME=chezscheme source build with either:
   • pack install (Idris2 package manager) — pulls a tested binary, fast install.
   • Pre-built tarball from idris-community/Idris2-pre-built-package or equivalent.
   • Setup action like stefan-hoeck/setup-idris2 if it exists.
2. Cache the Idris2 install across runs (currently it builds from source every time, ~7-10 min).
3. Remove continue-on-error: true from the job.

This isn't a docudactyl correctness issue — it's CI hygiene + faster feedback. The actual ABI proofs in src/Docudactyl/ABI/ are the strict gate worth recovering.

Related: see feedback_pre_compact_discipline / estate CI hygiene practices.