chore(examples): Apply RNRepo to FabricExample

[radoslawrolka]

Description

Adds RNRepo build-tools integration to FabricExample so that CI builds can consume pre-built native dependencies instead of compiling them from scratch, speeding up CI.

The integration is guarded by CI=true / ENV['CI'] environment variables, so local development is unaffected.

Changes

• FabricExample/android/build.gradle — resolves @rnrepo/build-tools via Node and adds its Gradle plugin JAR to the buildscript classpath.
• FabricExample/android/app/build.gradle — applies org.rnrepo.tools.prebuilds-plugin when running in CI.
• FabricExample/ios/Podfile — requires the RNRepo CocoaPods plugin and calls rnrepo_post_install in the post_install hook, both gated on ENV['CI'].
• FabricExample/package.json — adds @rnrepo/build-tools ~0.1.3-beta.0 as a dev dependency.

Performance

CI build time comparison (rnrepo vs baseline):

Platform	Category	rnrepo	basic	Faster
Android	Without runner cache	6:53 avg	8:35 avg	19.8%
Android	With runner cache	3:17 avg	5:29 avg	40.1%
iOS	Without runner cache	6:22	6:16	-1.6%
iOS	With runner cache	6:28 avg	6:18 avg	-2.6%
iOS	With runner cache and prebuilt rncore	3:31 avg	4:01 avg	12.4%

Android sees the biggest wins (~20% standard, ~40% with cache). iOS gains are visible when rncore is prebuilt (~12%) (tracked in #3955).

Test plan

• Verified that a local build (without CI=true) still works without any RNRepo-related side effects.
• CI run with CI=true should pick up pre-built artifacts via the RNRepo plugin on both Android and iOS.

[kkafar]

LGTM, thanks!