binding-tier-1: Crystal binding standup (replicate from Chapel pilot)

[hyperpolymath]

Parent: #129.

Scope

Apply the detachable-harness template from
docs/adr/0001-binding-ci-template.adoc to bindings/crystal/.

Detachability contract for the Crystal binding

The Crystal binding must compose its own detachable set:

• bindings/crystal/ — all sources, tests, scripts,
  symbol-manifest.txt, README, shard.yml, Justfile
• bindings/c/include/proven.h — vendored on extraction
• .github/workflows/crystal-ci.yml — workflow file, sparse-checkout
  pattern matching the Chapel pilot

The root Justfile gains crystal-{check,build,test,clean} as one-line
forwarders into bindings/crystal/Justfile. Nothing under
bindings/crystal/ may reach into the proven repo root or any
sibling binding.

Replication steps (mechanical)

1. Search/replace chapel → crystal, Chapel → Crystal,
   *.chpl → *.cr, Mason.toml → shard.yml, chpl --version
   → crystal --version across the Chapel template.
2. Adapt the WIRED extern list to the Crystal binding's current
   declarations.
3. Trim to the proven#88 wired subset
   (proven_path_has_traversal, proven_header_has_crlf,
   proven_free_string, proven_version, proven_build_info).
4. Replace the Chapel chpl --library typecheck command with the
   Crystal equivalent (crystal build --no-codegen).
5. Pin the Crystal toolchain by URL + SHA-256 in the workflow's
   env: block.
6. Land in a single PR titled feat(crystal-binding): stand up detachable CI + tests — binding-tier-1.

References

• Parent: proven: binding-tier-1 detachable-harness template — Chapel pilot + replication #129
• Pilot ADR: docs/adr/0002-chapel-binding-standup.adoc
• Template ADR: docs/adr/0001-binding-ci-template.adoc
• proven#88 — prerequisite ramp