build: pin Gradle daemon to JDK 21, add ROADMAP.md

[JMAN730]

Description

Two independent changes, both docs/build only — no app code is touched.

1. Pin the Gradle daemon to JDK 21

Contributors on a recent JDK hit ... is not compatible with Java 25 when running ./gradlew, and had to switch JAVA_HOME down to 21 by hand before every build. Gradle 8.10.2 cannot run on JDK 24 or newer — Gradle 9.1.0 is the first release that supports Java 25 — so this is a local-JDK problem, not a change in any package or plugin version.

Java 21 is already the intended toolchain across the project:

• app/build.gradle: sourceCompatibility / targetCompatibility VERSION_21, jvmTarget '21', jvmToolchain(21)
• .github/workflows/android-ci.yml: all three jobs use temurin 21

Changes:

• Add gradle/gradle-daemon-jvm.properties with toolchainVersion=21, so Gradle selects an installed JDK 21 for the daemon regardless of what JAVA_HOME points at. No more manual switching.
• Correct README.md and docs/development_guide.md, which advertised "JDK 21+". The missing upper bound is what made this look like a dependency-version bug.

A JDK 21 must still be installed locally — Gradle 8.10.2 will not provision one, so a machine with only JDK 25 now gets an explicit "no matching Java installation" error instead of a confusing incompatibility message.

Out of scope: building on JDK 25 natively would require Gradle 9.1+ and AGP 8.8.2 → 9.x, which is a real migration (removed Gradle 8 APIs such as rootProject.buildDir, deprecated kotlinOptions / packagingOptions, plus AGP 9 breaking changes). That belongs in its own PR — it is recorded in the roadmap below.

2. Add ROADMAP.md

Records where the project stands at 1.0.2 and what comes next, anchored to specific files and tickets rather than intentions. Every claim in it is checked against the tree rather than assumed. The two near-term items that carry a deadline or a correctness cost:

• targetSdk 36 before 2026-08-31. Google Play requires Android 16 for new apps and updates from that date (extension available through 2026-11-01). The project targets 35.
• Placeholder expectedSize values at app/src/main/java/com/opendroid/ai/core/llm/OnDeviceModelRegistry.kt:132 and :146 (marked TODO, 2 GB / 4 GB). Download integrity checks compare against these, so the LiteRT-LM path currently verifies against invented numbers.

It also snapshots health metrics — lint baseline size (49), unit-test classes (26), instrumentation tests (0, there is no androidTest source set) — so later drift is visible.

Fixes # (issue)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

No app code changed, so there is nothing to verify on a device — the build change affects only which JVM runs the Gradle daemon, and the rest is documentation.

• Test Configuration: Linux x64, JDK 21.0.10 (Temurin/Ubuntu), Gradle 8.10.2 wrapper.
• Manual Verification steps:
  1. Ran ./gradlew --version before the change → Daemon JVM: /usr/lib/jvm/java-21-openjdk-amd64 (no JDK specified, using current Java home).
  2. Ran ./gradlew --version after the change → Daemon JVM: Compatible with Java 21, any vendor (from gradle/gradle-daemon-jvm.properties), confirming Gradle 8.10.2 honors the criteria file.
  3. CI passed all three Gradle jobs with the criteria file in place (testDebugUnitTest + assembleDebug, lintDebug, unsigned assembleRelease), confirming it does not disturb daemon startup or toolchain resolution in a clean environment.
  4. A full Android build could not be run locally: dl.google.com is blocked by the sandbox network policy, so AGP and the AndroidX artifacts fail to resolve. CI is the validation path.
  5. Not verified: behavior with JDK 25 as JAVA_HOME. No JDK 25 could be installed in this environment (api.adoptium.net is also blocked), so the end-to-end "gradlew works while JAVA_HOME points at 25" path is unconfirmed and worth a check by whoever hit the original error.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new build errors or warnings
• I have updated the documentation in the docs/ folder (if applicable)

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[greptile-apps]

Confidence Score: 5/5

Safe to merge — no application code is changed; the new properties file uses the correct key and value for Gradle 8.10.2, and the documentation corrections are accurate.

The change is a single-property configuration file plus documentation updates. The toolchainVersion=21 key is the exact format shown in the Gradle 8.10.2 official docs, and the wrapper confirms the project is on 8.10.2. No build logic, Kotlin source, or CI workflow is modified, so there is nothing that could regress a build that was already passing.

Files Needing Attention: No files require special attention.

Important Files Changed

Filename	Overview
gradle/gradle-daemon-jvm.properties	New file pinning the Gradle daemon JVM to Java 21 using toolchainVersion=21; format is correct for Gradle 8.10.2 per official docs.
README.md	Prerequisite corrected from "JDK 21+" to "JDK 21 exactly" with accurate rationale explaining the Gradle 8.10.2 / JDK 24+ incompatibility.
docs/development_guide.md	Same JDK version correction as README with additional note that Gradle will not auto-provision the JDK.
ROADMAP.md	New living document added tracking current project state and upcoming work; references PR #50 as open (will be stale on merge, but expected for a living doc).

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A([Developer runs ./gradlew]) --> B{gradle-daemon-jvm.properties\nexists?}
    B -- No\n(before this PR) --> C[Use JAVA_HOME as daemon JVM]
    C --> D{JAVA_HOME ≥ JDK 24?}
    D -- Yes --> E[💥 'not compatible with Java 25'\nGradle 8.10.2 fails]
    D -- No --> F[✅ Daemon starts]
    B -- Yes\ntoolchainVersion=21\n(after this PR) --> G[Gradle toolchain resolver\nscans installed JVMs]
    G --> H{JDK 21 found?}
    H -- Yes --> I[✅ Daemon starts on JDK 21\nregardless of JAVA_HOME]
    H -- No --> J[❌ Explicit error:\n'no matching Java installation'\nfor version 21]

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_{Reviews (2): Last reviewed commit: "docs: add ROADMAP.md" | Re-trigger Greptile}