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>

Author: ericproulx

CHANGELOG.md

@@ -11,6 +11,7 @@
 * [#2687](https://github.com/ruby-grape/grape/pull/2687): Skip backtrace capture on internal validation exceptions - [@ericproulx](https://github.com/ericproulx).
 * [#2688](https://github.com/ruby-grape/grape/pull/2688): Consolidate user-registered rescue handler lookup into `Middleware::Error#registered_rescue_handler` backed by a shared `rescue_handler_from` primitive - [@ericproulx](https://github.com/ericproulx).
 * [#2689](https://github.com/ruby-grape/grape/pull/2689): Avoid empty-hash merges on request hot paths - [@ericproulx](https://github.com/ericproulx).
+* [#2690](https://github.com/ruby-grape/grape/pull/2690): Avoid allocating an empty array on every `StackableValues#[]` miss - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 #### Fixes

lib/grape/endpoint.rb

@@ -46,7 +46,7 @@ def initialize(new_settings, **options, &block)
       # this endpoint and its parents, but later it will be cleaned up,
       # see +reset_validations!+ in lib/grape/dsl/validations.rb
       inheritable_setting.route[:declared_params] = inheritable_setting.namespace_stackable[:declared_params].flatten
-      inheritable_setting.route[:saved_validations] = inheritable_setting.namespace_stackable[:validations]
+      inheritable_setting.route[:saved_validations] = inheritable_setting.namespace_stackable[:validations].dup
 
       inheritable_setting.namespace_stackable[:representations] ||= []
       inheritable_setting.namespace_inheritable[:default_error_status] ||= 500

lib/grape/util/stackable_values.rb

@@ -3,12 +3,14 @@
 module Grape
   module Util
     class StackableValues < BaseInheritable
-      # Even if there is no value, an empty array will be returned.
+      EMPTY = [].freeze
+
+      # Even if there is no value, an empty (frozen) array will be returned.
       def [](name)
         inherited_value = inherited_values[name]
         new_value = new_values[name]
 
-        return new_value || [] unless inherited_value
+        return new_value || EMPTY unless inherited_value
 
         concat_values(inherited_value, new_value)
       end