perf: abstract FormatCache as pluggable trait, optimize format runtime

Extract format string cache from static field in Format.scala into a
pluggable FormatCache trait (analogous to ParseCache). This allows users
to supply custom cache implementations (e.g., Caffeine-based) via the
Interpreter/Evaluator constructors.

Key changes:
- New FormatCache trait with getOrElseUpdate API
- DefaultFormatCache: LRU LinkedHashMap (256 entries), thread-safe
- FormatCache.SharedDefault singleton preserves process-wide sharing
- FormatCache.EmptyCache for testing
- CompiledFormat sealed trait for type-safe opaque cache entries
- RuntimeFormat: direct Val dispatch, Long fast path, pre-cached specs
- PartialApplyFmt pre-parses at construction time (no cache needed)
- FormatCache threaded through Interpreter → Evaluator constructors

Upstream: he-pin/sjsonnet jit branch (format optimization commits)

Author: He-Pin

sjsonnet/src-jvm-native/sjsonnet/SjsonnetMainBase.scala

@@ -373,7 +373,8 @@ object SjsonnetMainBase {
       logger = warnLogger,
       std = std,
       variableResolver = _ => None,
-      debugStats = debugStats
+      debugStats = debugStats,
+      formatCache = FormatCache.SharedDefault
     ) {
       override def createEvaluator(
           resolver: CachedResolver,

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -22,7 +22,8 @@ class Evaluator(
     val wd: Path,
     val settings: Settings,
     logger: Evaluator.Logger = null,
-    val debugStats: DebugStats = null)
+    val debugStats: DebugStats = null,
+    override val formatCache: FormatCache = FormatCache.SharedDefault)
     extends EvalScope {
   implicit def evalScope: EvalScope = this
   def importer: CachedImporter = resolver
@@ -1256,8 +1257,9 @@ class NewEvaluator(
     private val w: Path,
     private val s: Settings,
     private val wa: Evaluator.Logger = null,
-    ds: DebugStats = null)
-    extends Evaluator(r, e, w, s, wa, ds) {
+    ds: DebugStats = null,
+    fc: FormatCache = FormatCache.SharedDefault)
+    extends Evaluator(r, e, w, s, wa, ds, fc) {
 
   override def visitExpr(e: Expr)(implicit scope: ValScope): Val = try {
     (e.tag: @switch) match {