perf: use StringBuilderWriter across renderers and stdlib escape paths

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Motivation

Continues the synchronous java.io.StringWriter elimination started by PRs #875 (TomlRenderer) and #889 (std.deepJoin). Each StringWriter write goes through StringBuffer's synchronized monitor — pure overhead for the single-threaded code paths every renderer takes.

This PR landed in two parts:

1. The original commit covers std.escapeStringJson / std.escapeStringPython.
2. A follow-up commit extends the same StringBuilderWriter substitution across the remaining stdlib escape path (std.escapeStringXML), the YAML / PrettyYAML / Python / JSON renderers, and Renderer / PythonRenderer default out parameters (which transitively benefit Val.Obj.renderString, Format.scala's %r complex-type path, etc.).

Modification

Part 1: stdlib escape entry points

• std.escapeStringJson: for Val.Str input, call BaseRenderer.escape directly with a StringBuilderWriter, bypassing Materializer.stringify dispatch, Renderer allocation, and CharBuilder allocation. For non-string input, use StringBuilderWriter instead of java.io.StringWriter.
• std.escapeStringPython: same StringBuilderWriter substitution.

Part 2: rest of the renderer surface

• std.escapeStringXML: replace internal StringWriter with a pre-sized StringBuilderWriter.
• Renderer / PythonRenderer: default out parameter changes from java.io.StringWriter to StringBuilderWriter. Call sites that rely on the default (Val.Obj.renderString in Val.scala, Format.scala's complex-type %r path) inherit the optimization with no signature change.
• YamlRenderer: full visitor type-parameter migration from StringWriter to StringBuilderWriter; outBuffer switches from StringBuffer to StringBuilder (same length / setLength / charAt interface). StringBuilderWriter gains a small getBuilder accessor so YamlRenderer's trailing-space trim can still reach into the buffer.
• PrettyYamlRenderer: default out parameter to StringBuilderWriter. Also drops a dead StringWriter allocation in the quoted-string branch — the writer's result was never consumed (quotedStr is used instead).
• bench: MaterializerBenchmark.renderWith updated to construct a StringBuilderWriter to match the new constructor signatures.

Result

• Eliminates StringBuffer monitor acquisition on every char / string write across all remaining renderers (YAML / PrettyYAML / Python / default-JSON) and all three std.escapeString* entry points.
• Removes one dead StringWriter allocation per quoted-string in PrettyYamlRenderer.

Benchmarks (Scala Native, Apple M3 Pro)

std.escapeStringJson micro-benchmark via hyperfine:

Benchmark	Before (ms)	After (ms)	jrsonnet (ms)	Gap
std.escapeStringJson	7.0 ± 3.1	6.9 ± 0.7	4.6 ± 1.6	1.48×

Tests

• All 465 native tests + 48 JVM tests pass.