Add R8 -checkdiscard verification for disabled-flag dead code

[kirich1409]

Why

The plugin already causes dead-code elimination with generated -assumevalues rules, but nothing verifies the disabled-flag code actually left the final binary. As noted in the discussion that prompted this: unit tests see the full pre-R8 classpath, androidTest is assembled without minify, so nothing checks the final DEX. R8's built-in -checkdiscard directive is exactly the missing piece — it lists feature classes and fails the build if they survive optimization (Discard checks failed + the retention path; diagnose with -whyareyoukeeping).

Paired with -assumevalues this closes the loop: the flag is pinned to false, its branch and classes are stripped, and -checkdiscard confirms their absence. A forgotten DI reference, manifest entry, reflection, or over-broad -keep that keeps a "disabled" feature alive is now caught at build time instead of shipping.

What

A new opt-in discard(...) on the flag DSL plus a dedicated task, wired into R8 exactly like the existing -assumevalues machinery.

featured {
    localFlags {
        boolean("new_checkout", default = false) {
            discard("com.example.checkout.newflow.**")
        }
    }
}

generates:

# guarded by flag: new_checkout
-checkdiscard class com.example.checkout.newflow.** { *; }

Surface

• FlagSpec.discard(classSpec) — repeatable; rejected on non-boolean flags, on default = true flags, and on remote flags (a value never pinned at build time can't be guaranteed). Validation fails fast in the DSL with a precise message.
• CheckDiscardRulesGenerator — pure, testable rule generator (header + one -checkdiscard line per target).
• generateFeaturedCheckDiscardRules task → build/featured/proguard-featured-checkdiscard.pro. Reads discard targets straight from the DSL (config-cache safe, participates in the @Input fingerprint). The output is blank (a harmless no-op) unless a flag opts in.
• Wiring mirrors -assumevalues: into the app's own R8 (proguardFiles) for application modules, and as consumer ProGuard rules for library / KMP-library modules so the check runs in the consuming app's R8 — wherever the matching -assumevalues rule applies.

Tests

• CheckDiscardRulesGeneratorTest — output shape, trimming, blank-skipping, one rule per target.
• FlagDiscardDslTest — recording, descriptor serialization, and all three validation rejections.
• GenerateCheckDiscardRulesTaskRegistrationTest — task registration / group / wiring.
• R8CheckDiscardTest (real R8, in featured-shrinker-tests) — proves the check passes when the flag-guarded class is eliminated and fails the build (CompilationFailedException) when a checked class survives.

Docs

Updated root CLAUDE.md and featured-gradle-plugin/CLAUDE.md (DSL example, task table, discard(...) semantics and wiring notes).

Notes / scope

• Covers code only — -checkdiscard does not verify resource shrinking (call out in docs; resources still need aapt2/apkanalyzer inspection).
• No changes to the resolved-flags scan / manifest format — discard targets flow through their own provider, keeping blast radius small.
• Existing -assumevalues generation and wiring are untouched; the empty check-discard file is the same already-handled case as an empty -assumevalues file for flag-less modules.

Targets develop per the project's branching convention.

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