perf: direct long-to-chars rendering in visitFloat64

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Motivation

When rendering integer-valued doubles (e.g., 42.0), visitFloat64 currently calls i.toString which allocates a String object, then passes it through visitFloat64StringParts for character-by-character processing. For large arrays of numbers (common in Jsonnet), this creates millions of short-lived String allocations.

Key Design Decision

Add a writeLongDirect method that converts a Long directly into the CharBuilder's backing array without any intermediate String allocation. Uses a digit-pair lookup table for two-digits-at-a-time conversion (a well-known optimization from JDK's Integer.getChars).

Modification

sjsonnet/src/sjsonnet/BaseCharRenderer.scala:

• Added writeLongDirect(v: Long) private method:
  • Special-cases 0 and Long.MinValue (negation overflow)
  • Counts digits, then writes right-to-left using digit-pair tables
  • Directly writes into elemBuilder.arr via ensureLength + position update
• Added companion object BaseCharRenderer with DIGIT_TENS and DIGIT_ONES lookup tables (100 entries each)
• Changed visitFloat64: integer path now calls writeLongDirect(i) instead of visitFloat64StringParts(i.toString, ...)

Test: Added new_test_suite/large_integer_rendering.jsonnet — verifies correct rendering of boundary values (0, negatives, large longs, Long.MinValue).

Benchmark Results

JMH — Full Suite (35 benchmarks, 1+1 warmup)

Pending benchmark data.

Expected Impact

Primarily benefits benchmarks with heavy numeric rendering:

• realistic2 (large JSON with many numbers)
• base64DecodeBytes (byte arrays rendered as numbers)
• Any workload generating numeric arrays

Analysis

• Correctness: Handles all edge cases — zero, negative, Long.MinValue overflow, single/multi-digit numbers.
• Digit-pair table: Converts two digits per iteration (100x div/mod instead of 10x), halving the number of divide operations.
• Zero allocations: No String or StringBuilder — writes directly into the backing char[].
• Regression test: large_integer_rendering.jsonnet covers boundary values including Long.MinValue.

References

• JDK's Integer.getChars uses the same digit-pair technique
• BaseCharRenderer.DIGIT_TENS / DIGIT_ONES — static lookup tables

Result

Zero-allocation integer rendering with digit-pair optimization. Eliminates Long.toString allocation overhead. Draft PR pending benchmark data.

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Superseded by #730 which combines this optimization with the other renderer throughput improvements (indent cache + bulk copy + direct long rendering) into a single coherent PR with comprehensive benchmarks.