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Lazy-allocate `@new_values` in `Grape::Util::BaseInheritable` (#2739)

`BaseInheritable#initialize` used to eagerly allocate an empty
`@new_values = {}` per instance. Most settings layers in a typical API
only inherit from a parent and never write any *new* values of their
own, so the empty Hash was just retained dead weight.

Defer the allocation: `@new_values` starts nil, the `[]=` writers do
`(@new_values ||= {})[name] = ...`, and the read paths in
`InheritableValues#[]` / `StackableValues#[]` short-circuit on nil so
they skip the lookup entirely instead of touching an empty Hash.
`initialize_copy` skips the `.dup` when the source has no own values.

Measured on master with a small benchmark:

  Grape::Util::InheritableSetting.new ×100
    before: 1600 objects, 183.6 kB
    after:  1200 objects, 121.1 kB   (-34%)

  parent.point_in_time_copy ×100
    before: 2500 objects, 277.3 kB
    after:  1700 objects, 152.3 kB   (-45%)

The bigger win on `point_in_time_copy` matches the
`base_inheritable.rb:27` (`new_values.dup`) hot spot in the boot-time
memory profile: that line previously dup'd four empty Hashes per copy
and now skips them entirely.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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