Cache columns_digest on class

[hmcguire-shopify]

We previously improved the speed of columns_digest by not allocating an intermediate array. However, columns_digest is still very hot in our application (~15% of time spent inside MessagePack::Unpacker#full_unpack).

This commit caches columns_digest on the model class by hooking into schema loading. Since the value is calculated from a model's columns_hash, this ensures that if the schema is reloaded the value is recalculated.

---

The benchmark in #67 shows a 2x speedup on dumping and loading an Active Record

$ ruby --yjit benchmark/active-record-unpack.rb
-- create_table(:records)
   -> 0.0110s
=== dump ===
ruby 4.0.2 (2026-03-17 revision d3da9fec82) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
        dump (after)    20.603k i/100ms
Calculating -------------------------------------
        dump (after)    204.449k (± 0.4%) i/s    (4.89 μs/i) -      1.030M in   5.038761s

Comparison:
       dump (before):   104609.9 i/s
        dump (after):   204449.0 i/s - 1.95x  faster


=== load ===
ruby 4.0.2 (2026-03-17 revision d3da9fec82) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
        load (after)    22.651k i/100ms
Calculating -------------------------------------
        load (after)    226.690k (± 0.4%) i/s    (4.41 μs/i) -      1.155M in   5.096033s

Comparison:
       load (before):   110229.1 i/s
        load (after):   226689.9 i/s - 2.06x  faster

[Chris911]

@hmcguire-shopify @chrisbutcher any blockers to landing this?

[chrisbutcher]

🎉 🙏