[OskarEichler]

Problem

The definite-array decoder allocated Ruby array capacity from the CBOR-declared element count before reading any elements. A short sequence of nested array headers can therefore reserve substantial capacity even when the input ends immediately. The existing change in this PR corrects the cap calculation, but a per-array cap alone does not limit the cumulative reservation across nested arrays.

On Ruby 4.0.6, 32 nested three-byte headers (96 input bytes) reserved 16,778,240 bytes of Ruby Array capacity before EOFError. This is an allocation measurement, not a demonstrated resident-memory increase or process crash.

Fix

Create definite arrays empty and let rb_ary_push grow them as elements arrive. Retain the capped preallocation for raw strings. Decoded values, streaming behavior, and parser errors remain the same; valid large arrays may do additional proportional growth.

Verification

• rbenv exec rake compile and rbenv exec rake spec: 171 examples, 0 failures on Ruby 4.0.6, arm64 macOS.
• The 96-byte malformed input reserved 1,280 bytes of Array capacity after this change. A 160-byte variant using five-byte length headers also reserved 1,280 bytes.
• A valid 70,000-element array decoded correctly in one shot and through incremental feeding.
• The new MRI regression example checks the malformed nested-array allocation and preserves EOFError.