perf: abstract FormatCache as pluggable trait, optimize format runtime (#679)

## Motivation

The `%` format operator in Jsonnet is commonly used in string-heavy
workloads (e.g., realistic2). Each format call re-parses the format
string, which is wasteful when the same format pattern is used
repeatedly.

## Key Design Decision

Abstract format string parsing into a `FormatCache` trait, allowing
parsed format specifications to be cached and reused across calls. The
default implementation provides a simple LRU-style cache.

## Modification

- Add `FormatCache` trait with pluggable cache strategy
- Cache parsed format specifications keyed by format string
- Optimize format runtime dispatch for common patterns
- ~317 lines changed

## Benchmark Results

### JMH (JVM, 3 iterations warmup + 3 measurement)

| Benchmark | Master (ms/op) | This PR (ms/op) | Change |
|-----------|---------------|-----------------|--------|
| bench.02 | 50.427 ± 38.9 | 45.984 ± 2.4 | **-8.8%** |
| comparison2 | 85.854 ± 188.7 | 69.734 ± 19.9 | **-18.8%** |
| realistic2 | 73.458 ± 66.7 | 70.857 ± 3.2 | **-3.5%** |

## Analysis

The format cache helps most on comparison2 (-18.8%) which has repeated
format patterns. The realistic2 improvement is modest (-3.5%) because
its format patterns have more variety. The infrastructure enables
further optimizations on format-heavy workloads.

## References

- Upstream: jit branch experiment

## Result

All 5 tests pass. All benchmarks positive, no regressions.

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
@@ -1270,8 +1271,9 @@ 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 {