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Return frozen EMPTY array from StackableValues on miss (#2690)

`StackableValues#[]` is the accessor behind `namespace_stackable[key]` —
called multiple times per request (five filter phases, plus ad-hoc
settings lookups). When neither `inherited_values` nor `new_values`
have an entry for the key, the old `return new_value || []` allocated
a fresh empty array on every miss.

Introduce a frozen `EMPTY` singleton and return it in the miss branch.
Saves two short-lived empty-array allocations per request in the
realistic shape (some filters set, some not) and up to five in the
worst case.

One caller (`Endpoint#initialize`) stored `namespace_stackable[:validations]`
directly into `route[:saved_validations]` by reference, and later
`.concat`'d parent validations into it via `inherit_settings`. With the
stackable now able to return a frozen array, that mutation would
`FrozenError`. Dup at capture so the route keeps its own mutable list —
good hygiene regardless of this change.

Realistic (3 keys populated, 2 absent):
  current: 2.01 M i/s, 80B / 2 objects allocated
  variant: 2.19 M i/s, 0B / 0 objects allocated

Worst case (all 5 absent):
  current: 2.18 M i/s, 200B / 5 objects
  variant: 2.49 M i/s, 0B / 0 objects  (1.14x faster)

No behavior change; all 2,236 specs pass.

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

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