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**Persistent javac/kotlinc compilation daemon for `ya make` (opt-in)**
**Note:** the code in this PR was generated mostly by an LLM.
This PR adds an opt-in persistent compilation daemon that runs `javac` and `kotlinc` in-process inside a long-lived JVM, instead of spawning a fresh subprocess per build node. Each Java/Kotlin module today pays the full JVM \+ compiler startup cost (\~150–340 ms of startup, plus \~55 MB of kotlinc class-loading) on every node, which dominates the wall time of short compiles; amortizing that startup across all nodes in a build session yields measurable speedups (1.3x–3.7x CPU economy on the benchmarked targets, with zero correctness fallbacks or failed nodes).
The daemon (`devtools/buildstep_tools/javac_daemon`, a `JAVA_PROGRAM` built to a committed `JavacDaemon.jar`) listens on a Unix domain socket and runs both compilers concurrently; a thin Python client in the existing build scripts (`javac_daemon_client.py`, wired into `run_javac.py` / `compile_java.py` via `build/conf/java.conf`) decides per-node eligibility, routes launcher flags, selects/launches the right daemon instance, and **always falls back to a plain subprocess** on any doubt (old JDK, resource JVM flag, denied annotation processor, daemon error), so build correctness never depends on the daemon. The feature is **off by default** and activated only via `YA_JAVAC_DAEMON=1` passed to `ya make`; without it the build uses the existing subprocess path unchanged.
See `junk/iniklyaev/javac-daemon/DESIGN_javac_daemon.md` for the full design, flag-routing rules, fault-isolation model, and benchmark methodology.
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