feat: enable gamma project by default

[kkafar]

Description

Flip the gamma-project gate from opt-in to opt-out on both platforms.
Previously gamma sources were included only when the env var / Gradle
property was explicitly set to a truthy value; now they are included
by default and excluded only when the value is explicitly set to the
disable sentinel.

This makes gamma the default for downstream consumers and CI, while
still giving users a clean escape hatch if they need the pre-gamma
build.

The main motivation behind the change is to allow downstream solutions
exposing our experimental APIs w/o additional setup steps for the
end users.

I want to also note, that this should be safe to do, because the
synchronous event APIs are available since 0.82, which is the currently
oldest supported RN version.

Important

If any reviewer is aware of any React Native API we use in gamma code,
that is not available in 0.82, please point it out.

Changes

• iOS (RNScreens.podspec): RNS_GAMMA_ENABLED switched from
  == '1' opt-in to != '0' opt-out. Set RNS_GAMMA_ENABLED=0 to
  disable.
• Android (android/build.gradle): rnsGammaEnabled switched from
  == "true" opt-in to != "false" opt-out. Set
  rnsGammaEnabled=false (e.g. in gradle.properties or
  -PrnsGammaEnabled=false) to disable.
• Added one-line comments on both sides documenting the opt-out
  semantics.

Test plan

No new tests — this is a build-config default change. Manual verification:

• iOS, default (no env set): pod install in FabricExample/ios
  should log [RNScreens] Gamma project enabled. Including source files.
  and include ios/gamma/** sources.
• iOS, explicit disable: RNS_GAMMA_ENABLED=0 pod install should
  exclude ios/gamma/** and instead pull in ios/stubs/**.
• Android, default (no property): BuildConfig.RNS_GAMMA_ENABLED
  resolves to true.
• Android, explicit disable: ./gradlew ... -PrnsGammaEnabled=false
  resolves BuildConfig.RNS_GAMMA_ENABLED to false.

Follow-ups (not in this PR):

• FabricExample/android/app/build.gradle:91 still uses the old
  opt-in check and will need a matching flip if the example app
  should track the library default.
• CI workflows still pass RNS_GAMMA_ENABLED: 1 (now redundant); no
  job currently exercises the disabled path.
• guides/CONTRIBUTING.md:128 documents the old opt-in flow and
  needs updating.
• Stale podspec comment about DEFINES_MODULE / React-RCTImage
  will be removed in a separate PR.

Important

Before merging I need to verify build on 0.82
We need to also establish whether we'll want to use any APIs above 0.82 in near future. > If so, we can not turn this flag on.

Checklist

• Included code example that can be used to test this change.
• For visual changes, included screenshots / GIFs / recordings documenting the change.
• For API changes, updated relevant public types.
• Ensured that CI passes

[kkafar]

cc @satya164

[kkafar]

I'll wait a bit with landing this one.

It might turn out problematic, because we assume no backward compatibility requirement for the new major, and in case we want to use any of the new React Native's APIs, such as optional values in codegen introduced in 0.84, we'll break the builds of the stable version.

We need to either think of some new solution OR:

1. require anyone wanting to test the new features to enable appropriate feature flags,
2. do a branch cut and require anyone wanting to test the new features to update the package.

Currently I'm not sure middle ground is possible.