perf: cached renderDouble for small integers + Builtin1/3 apply overrides

[He-Pin]

Motivation

RenderUtils.renderDouble is called on every number materialization. Small integer values (0-255) are common in Jsonnet workloads (array indices, ASCII codepoints, counters), so caching their string representations avoids repeated Long.toString allocation.

Builtin1 and Builtin3 were also missing direct apply1 / apply3 overrides while Builtin2 already had apply2, leaving avoidable intermediate Array allocation on those builtin calls.

Key Design Decision

1. Cache string representations for integer values 0-255 in RenderUtils.
2. Reuse renderDouble from Materializer.stringify so number rendering has one fast path.
3. Add direct apply1 and apply3 overrides for Builtin1 and Builtin3, matching the existing Builtin2.apply2 pattern.

Modification

• Renderer.scala: add a small integer string cache and use it when rendering non-negative integral doubles in range 0-255.
• Materializer.scala: delegate numeric stringification to RenderUtils.renderDouble instead of duplicating integer handling.
• Val.scala: add apply1 override to Builtin1 and apply3 override to Builtin3.

Benchmark Results

No benchmark was remeasured in this rebase/test cycle. Targeted benchmark work is deferred to the later serial benchmark batch.

Prior directional evidence from this PR:

Benchmark	Master (ms/op)	This PR (ms/op)	Change
manifestTomlEx	0.074	0.069	-6.8%
realistic2	57.139	58.868	+3.0% noise
large_string_template	1.610	1.646	+2.2% noise

Prior wider JMH sweep summary: 7 improvements, 8 regressions, 20 neutral on Apple Silicon single-shot average. Treat as directional only.

Analysis

The integer cache removes allocation for the hot 0-255 rendering range while preserving the existing rendering semantics for other numbers. The builtin apply overrides remove avoidable argument-array allocation and follow an existing code pattern. The current cycle was limited to safe rebase, formatting, full test validation, PR body normalization, and protected branch update.

References

• Upstream source commit: 9917a682
• Rebased onto upstream/master fd1b3b1e

Result

Rebase completed cleanly onto upstream/master. Validation in this cycle:

• rtk ./mill __.reformat: SUCCESS; worktree remained clean.
• rtk ./mill __.test: SUCCESS.