fix: Only treat frozen Numeric defaults as immutable

[jterapin]

Issue #:

• ItemData#clean! is O(attributes^2) in Marshal round-trips (populate_default_values called inside the per-attribute loop) #158 (follow-up)

Description of changes:
The _immutable? fast path added in #159 used value.is_a?(Numeric), which matches any Numeric subclass. A mutable custom Numeric subclass used as a default_value was therefore returned by identity and shared across every record in the process, so mutating it on one record leaked into the others.

This guards the numeric branch with frozen?: only genuinely immutable numerics skip the deep copy. Every stdlib numeric is already frozen, so they stay on the fast path and the #159 optimization is preserved; an unfrozen mutable subclass falls back to the existing deep copy.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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Written with AI assistance and reviewed by jterapin