feat(surveys): add posthog-android-surveys-compose default UI module

[leonhardprinz]

Context

Closes the long-standing gap in #102 (open since Feb 2024): the Android SDK's default surveys delegate is a stub that only logs. Customers using surveys on Android have to implement their own UI, which is a significant lift.

Driven by large customer (20+ global apps), who are blocked rolling out post-print NPS surveys on Android. iOS shipped a complete SwiftUI implementation in PostHog/posthog-ios#320 — this PR ports the same pattern to Kotlin/Compose as an opt-in module so non-Compose apps don't pay for it.

Disclosure: I'm a TAM, not an engineer. This is vibe-coded with PostHog Code using iOS PR #320 as the port-from blueprint. Please push back hard on any architectural calls that conflict with SDK conventions.

What's in this PR

• New module :posthog-android-surveys-compose — Material 3 ModalBottomSheet + Jetpack Compose
• NPS / Number Rating question type only — Fujifilm's primary use case (post-print NPS)
• Theming sourced from PostHogDisplaySurveyAppearance — background, submit button, text, border, rating-button colors all pull through, plus a faithful port of the iOS hex / CSS-color name parser (140-entry table)
• Lifecycle callbacks wired — onSurveyShown / onSurveyResponse / onSurveyClosed fire at the right moments so the SDK's survey shown / survey sent / survey dismissed events fire correctly. The delegate never calls PostHog.capture directly.
• ComposeView injected into the foreground Activity's android.R.id.content via an ActivityLifecycleCallbacks-backed ActivityProvider — works on any Activity type (no FragmentActivity requirement)
• @Preview composables for SurveySheet / NumberRating / BottomSection in default and themed appearances
• Sample app wired — Trigger test survey button capturing show_test_survey, delegate set in MyApp.kt

What's NOT in this PR (planned follow-ups)

• Open text, single-choice, multiple-choice, and link question types (placeholder rendered today so the sheet doesn't crash)
• Emoji rating
• Thank-you / confirmation screen (displayThankYouMessage == true path)
• Branching logic verification across multi-question surveys
• Dark-mode polish
• Compose UI tests + accessibility audit

Architecture decisions

• Separate Gradle module (sibling to :posthog-android, no changes to :posthog or :posthog-android) — non-Compose apps don't pay APK-size cost. Mirrors the AndroidX optional-Compose-module pattern.
• Independent versioning starting at 1.0.0-alpha01 — lets this module iterate through alphas without dragging core SDK versions. Module added to binary-compatibility-validator ignoredProjects during alpha.
• Compose BOM 2024.12.01 (matches the sample app's existing pin), Material 3 1.3.1, kotlin.plugin.compose for K2-era Compose.
• rememberModalBottomSheetState(confirmValueChange = { it != SheetValue.Hidden }) for the "X-button-only dismissal" — matches iOS interactiveDismissDisabled() semantics with one line instead of a custom SheetState.
• Empty consumer-rules.pro + -keep PostHogSurveysComposeDelegate — minimal consumer-side ProGuard surface; populate if downstream minified builds ever surface issues.
• @file-level ktlint_function_naming_ignore_when_annotated_with = Composable in module .editorconfig so PascalCase @Composable functions don't trip the standard:function-naming rule.

Local verification

This is what I ran in my dev env (no Android emulator available there, so the visual checks are deferred to a real device run):

• ./gradlew spotlessCheck — passes
• ./gradlew detekt — passes
• ./gradlew :posthog:build — passes (confirming no regression in core)
• Android-specific tasks (:posthog-android-surveys-compose:assembleDebug, :posthog-android-surveys-compose:lint, full ./gradlew build) deferred to CI here because my dev env doesn't have the Android SDK

Manual test plan (for me to run on an emulator before un-drafting)

1. Build & install the sample app on a Pixel emulator (API 34)
2. In a PostHog dev project, create a Popover survey with an NPS question, target it on show_test_survey, set custom background/submit colors so theming is visible
3. Tap "Trigger test survey" in the sample app
4. Verify: sheet slides up, scale renders 0–10, tap-to-select highlights with submit color, tap same number deselects, submit fires survey sent with the right rating, X-button fires survey dismissed, swipe-down is ignored

Reviewers

cc @marandaneto @ioannisj @lucasheriques

Will also link the Feb 2026 internal Slack thread on Android surveys in a comment.

Refs #102