feat(jdtls): support upstream JDTLS via jdtls_path/lombok_path settings

CHANGELOG.md

@@ -49,6 +49,7 @@ Status of the `main` branch. Changes prior to the next official version change w
   - Fix: Lean4 stale cache — empty document symbol responses (returned before `lake build` completes) are no longer persisted, preventing symbols from being permanently hidden #1356
   - Add JSON language server support via `vscode-json-languageserver` (experimental) #1391
   - Fix: Elixir/Expert deadlock on startup — Expert's build pipeline requires a `textDocument/didOpen` notification to start; Serena now opens `mix.exs` immediately after `initialized` so Expert begins compiling instead of waiting indefinitely #1397
+  - Java (`eclipse.jdt.ls`): Add upstream JDTLS mode for offline / restricted-network use. Setting both `jdtls_path` and `lombok_path` in `ls_specific_settings.java` makes Serena use an existing upstream JDTLS installation (e.g. `brew install jdtls`) and the system JDK 21+, skipping the ~500 MB vscode-java VSIX, Gradle, and IntelliCode downloads. New related setting `java_home` lets the user override the JDK used to launch JDTLS. Default behavior unchanged — the JDTLS workspace hash is preserved bit-for-bit for users on the default route, so existing project caches are reused without a one-time reindex; the launcher path is mixed into the hash only when `jdtls_path` is set, isolating upstream installations from the default workspace. #1415
 
 Dashboard:
   - Add configurable dashboard interface mode (new global configuration setting `web_dashboard_interface`):

docs/02-usage/050_configuration.md

@@ -504,28 +504,72 @@ ls_specific_settings:
 
 #### Java (`eclipse.jdt.ls`)
 
+Java support has two installation modes:
+
+1. **Default vscode-java VSIX mode** (no extra config required): Serena downloads the platform-specific
+   vscode-java VSIX (~500 MB: JDTLS + bundled JRE 21 + Lombok + IntelliCode), Gradle distribution and
+   IntelliCode VSIX from public hosts on first use.
+2. **Upstream JDTLS mode** (offline-friendly): Activated by setting both `jdtls_path` and `lombok_path`.
+   Uses an existing JDTLS installation (~100 MB) and the system JDK 21+. Nothing is downloaded.
+   Recommended for restricted-network/corporate environments.
+
+**When to use which mode:**
+
+- **Default vscode-java VSIX mode** — recommended for most users. No setup required;
+  Serena downloads everything on first use.
+- **Upstream JDTLS mode** — recommended when:
+  - you cannot reach `github.com`, `services.gradle.org` or `marketplace.visualstudio.com`
+    from the host (corporate proxy, air-gapped network);
+  - you want a smaller on-disk footprint (~100 MB vs ~500 MB);
+  - you already maintain a JDTLS installation (e.g. for `nvim-jdtls` or another editor);
+  - your security policy prohibits per-project runtime downloads.
+
+**JDK 21+ is required** in upstream mode. Serena resolves the JDK in this order:
+`ls_specific_settings.java.java_home` → `JAVA_HOME` env var → first `java` on `PATH`.
+The resolved JVM is interrogated and rejected if its `java.specification.version` is below 21.
+
 The following settings are supported for the Java language server:
 
 | Setting | Default | Description |
 |---|---|---|
+| `jdtls_path` | `null` | Activates upstream JDTLS mode. Path to upstream JDTLS root (containing `plugins/` and `config_<platform>/`). Get via `brew install jdtls` or extract `jdt-language-server-*.tar.gz` from <https://download.eclipse.org/jdtls/snapshots/>. Must be set together with `lombok_path`. |
+| `lombok_path` | `null` | Path to the Lombok jar. Activates upstream JDTLS mode together with `jdtls_path`. Get from `~/.m2/repository/org/projectlombok/lombok/<ver>/lombok-<ver>.jar` or download from <https://projectlombok.org/downloads/>. |
+| `java_home` | `null` | (upstream-jdtls mode only) Path to JDK 21+ home directory used to launch JDTLS. Falls back to `JAVA_HOME` env var, then `which java`. |
 | `maven_user_settings` | `~/.m2/settings.xml` | Path to Maven `settings.xml` |
 | `gradle_user_home` | `~/.gradle` | Path to Gradle user home directory |
 | `gradle_wrapper_enabled` | `false` | Use the project's Gradle wrapper (`gradlew`) instead of the bundled Gradle distribution. Enable this for projects with custom plugins or repositories. |
 | `gradle_java_home` | `null` | Path to the JDK used by Gradle. When unset, Gradle uses the bundled JRE. |
 | `use_system_java_home` | `false` | Use the system's `JAVA_HOME` environment variable for JDTLS itself. Enable this if your project requires a specific JDK vendor or version for Gradle's JDK checks. |
-| `gradle_version` | `8.14.2` | Override the Gradle distribution version Serena downloads by default. |
-| `vscode_java_version` | `1.42.0-561` | Override the bundled `vscode-java` runtime bundle version Serena downloads by default. |
-| `intellicode_version` | `1.2.30` | Override the IntelliCode VSIX version Serena downloads by default. |
+| `gradle_version` | `8.14.2` | (vscode-java mode only) Override the Gradle distribution version Serena downloads by default. |
+| `vscode_java_version` | `1.42.0-561` | (vscode-java mode only) Override the bundled `vscode-java` runtime bundle version Serena downloads by default. |
+| `intellicode_version` | `1.2.30` | (vscode-java mode only) Override the IntelliCode VSIX version Serena downloads by default. |
 | `jdtls_xmx` | `3G` | Maximum heap size for the JDTLS server JVM. |
 | `jdtls_xms` | `100m` | Initial heap size for the JDTLS server JVM. |
-| `intellicode_xmx` | `1G` | Maximum heap size for the IntelliCode embedded JVM. |
-| `intellicode_xms` | `100m` | Initial heap size for the IntelliCode embedded JVM. |
+| `intellicode_xmx` | `1G` | (vscode-java mode only) Maximum heap size for the IntelliCode embedded JVM. |
+| `intellicode_xms` | `100m` | (vscode-java mode only) Initial heap size for the IntelliCode embedded JVM. |
 
-Note:
+Notes:
 - When overriding `vscode_java_version`, Serena still assumes that the downloaded runtime bundle keeps the same internal
   directory layout and file names as the bundled default version.
+- In upstream-jdtls mode, IntelliCode is not loaded (it's an ML completions ranker that is irrelevant to Serena's
+  symbol-tools workflow), and Serena does not ship a Gradle distribution. Maven projects work via JDTLS's bundled m2e.
+  Gradle projects must have `./gradlew` in the project, or rely on a system-installed Gradle through Buildship's
+  default discovery rules.
+- In upstream-jdtls mode the `gradle_version`, `vscode_java_version`, `intellicode_version`,
+  `intellicode_xmx`, `intellicode_xms` settings are silently ignored — they only apply to the
+  vscode-java VSIX mode.
+
+Example: upstream-jdtls mode (offline / corporate network):
+
+```yaml
+ls_specific_settings:
+  java:
+    jdtls_path: "/opt/homebrew/Cellar/jdtls/1.50.0/libexec"
+    lombok_path: "/Users/me/.m2/repository/org/projectlombok/lombok/1.18.36/lombok-1.18.36.jar"
+    # java_home: "/opt/homebrew/opt/openjdk@21"  # optional
+```
 
-Example for a project with custom Gradle plugins and JDK requirements:
+Example: default vscode-java VSIX mode for a project with custom Gradle plugins:
 
 ```yaml
 ls_specific_settings: