Harden generic-enum fixtures against layout and refcount bugs

[Kyle-Ye]

Summary

GestureOutputCompatibilityTests and GesturePhaseCompatibilityTests each
build their cases via a make(tag:_:) helper that (a) writes case fields at
hand-computed offsets and (b) extracts the value with load(as:) + defer deallocate. Two latent bugs made iOS Simulator discovery crash at xctest
bootstrap:

1. Offset bug. .empty(GestureOutputEmptyReason, GestureOutputMetadata?)
   wrote metadata at stride(GestureOutputEmptyReason) == 1, but the case's
   tuple layout aligns GestureOutputMetadata? (alignment 8) to offset 8.
   The metadata bytes landed in the payload's padding region; reading the
   value back tripped initializeWithCopy during Swift Testing's test
   discovery (xctest at initializeWithCopy for GestureOutput /
   xctest at static GestureOutput.make(tag:_:)). The same failure mode is
   latent in GesturePhase.blocked(value:blockedBy:) for any Value whose
   stride does not match GestureNodeID's alignment — it only worked
   because the tests used Value = Int.

2. Refcount leak. load(as:) bumps the returned value's retain count
   while deallocate() skips destroying the memory — whenever the payload
   carried a bridgeObject field, the retain initializeMemory wrote in
   place was leaked. The test values used so far are POD, so it was
   invisible until someone tested a String-carrying Value.

Both fixtures now share a tuple-based make<Payload>(tag:payload:):

• The payload is typed as a Swift tuple ((reason, metadata),
  (value, blockedBy), () for no-payload cases) so the compiler computes
  the correct field offsets and alignment padding for every case.
• Allocation uses UnsafeMutablePointer<…>.allocate(capacity:) + move(),
  transferring ownership out of the scratch slot instead of the leaky
  load+deallocate dance.

Also adds .active/.blocked tests for GesturePhase<String> and
.value/.finalValue tests for GestureOutput<String> to exercise a
bridgeObject-carrying payload through initializeWithCopy — these would
trip the leak under the old pattern.

Test plan

• xcodebuild test -only-testing:OpenGesturesTests on iPhone 17 Pro / iOS 26.2 Simulator
• OPENGESTURES_COMPATIBILITY_TEST=0 xcodebuild test -only-testing:OpenGesturesCompatibilityTests on the same simulator
• OPENGESTURES_COMPATIBILITY_TEST=1 OPENGESTURES_USE_LOCAL_DEPS=1 xcodebuild test -only-testing:OpenGesturesCompatibilityTests on the same simulator