ci: 'Rust Build — Real Chapel Library' fails to link (undefined chapel_* symbols) #408

The allow-fail job Rust Build — Real Chapel Library (allow-fail, L2.3+ gate) in chapel-ci.yml is red on main (b761b3a) and on #407. It is not caused by #407.

Failure. cargo build --features chapel against the real Chapel library fails at link time. rust-lld reports undefined symbols that are referenced from src/zig_ffi/chapel_bridge.zig (lines 209–292): chapel_is_available, chapel_parallel_search, chapel_free_result, chapel_get_version, chapel_available_prover_count, chapel_is_prover_available. The Zig archive libechidna_chapel_ffi.a is linked, but the Chapel-compiled object that defines these exports is not on the link line.

Acceptance criteria

• The job links and runs on a PR head, so the Chapel library is actually linked rather than stubbed.
• Or, if the L2.3+ gate is meant to stay unbuilt for now, the job is skipped with a stated reason, so it does not report red on every PR.
• docs/wikis/Home.md capability row "Chapel parallel search" is updated to match.

Deferred from #407 under AGENTS.md §5c item 3.