ci: build qs2/stringfish from source (RcppParallel 6.2.0 oneTBB ABI break)

[seonghobae]

Since ~2026-08-01, every kaefa CI run fails at package load on all platforms:

unable to load shared object '.../stringfish/libs/stringfish.so':
undefined symbol: _ZN3tbb8internal25concurrent_vector_base_v316internal_grow_byEmmPFvPvPKvmES4_

Root cause: RcppParallel 6.2.0 migrated to oneTBB and dropped the legacy tbb::internal symbols. The prebuilt CRAN/PPM binaries of qs2 and stringfish (pulled transitively via mirt → SimDesign → qs2) still link those symbols, and restored package caches carry the stale builds. Confirmed identical failures on macOS (Symbol not found: __ZN3tbb8internal12NFS_AllocateEmmPv), Windows (LoadLibrary failure: The specified procedure could not be found), and Ubuntu — pre-existing on all recent PR branches (e.g. runs 30878110721, 30876579843), unrelated to their diffs.

Fix: in all three workflows' setup-r-dependencies, force stringfish=?source and qs2=?source so they compile against the current RcppParallel, and bump cache-version to 2 to discard the poisoned caches. Revert both once CRAN/PPM rebuild the binaries against RcppParallel 6.2.0.

Unblocks #74 and every other open kaefa PR.

🤖 Generated with Claude Code

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