perf: preserve ASCII-safe simple format results

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Motivation:
large_string_template still spent time re-encoding and re-scanning the huge string produced by simple named format interpolation, even when the static format literals and dynamic values made the final string JSON-string ASCII-safe. This keeps the focus on the largest remaining local gap from #666-style benchmarking.

Key Design Decision
Track ASCII safety as metadata on the compiled simple named format path instead of adding another renderer scan. The optimization is deliberately limited to all-simple %(key)s formats, where every emitted dynamic value goes through simpleStringValue and can be conservatively classified.

Modification:

• Add staticAsciiSafe metadata to compiled format strings.
• Return Val.Str.asciiSafe from Format.PartialApplyFmt when static literals and all simple named dynamic values are JSON-string ASCII-safe.
• Keep unsafe strings, unsafe static literals, and mixed-key unsafe values conservative.
• Add focused regression tests for safe numeric values, unsafe dynamic strings, unsafe static literals, and mixed-key safety.
• Update the gap and sync ledgers.

Benchmark Results:
JVM JMH (bench.runRegressions bench/resources/cpp_suite/large_string_template.jsonnet):

• Before: 0.683 ms/op
• After: 0.677 ms/op

Scala Native hyperfine, large_string_template, before vs after:

• Forward: 8.64 +/- 0.75 ms -> 8.01 +/- 0.52 ms
• Reverse: 8.65 +/- 0.50 ms -> 8.17 +/- 0.48 ms

Scala Native hyperfine, source-built jrsonnet comparison:

• sjsonnet after: 8.01-8.17 ms
• jrsonnet: 6.0 +/- 1.2 ms
• Remaining gap: about 1.34x

Guard benchmark, kube-prometheus:

• Forward: 131.74 ms -> 129.14 ms
• Reverse: 129.11 ms baseline vs 130.94 ms candidate
• Interpreted as neutral/noisy, not a target regression.

Analysis:
The existing renderer already has an ASCII-safe direct byte path, but formatted strings lost that metadata and fell back to UTF-8 encoding plus escape scanning. This change preserves the metadata at the producer where the safety condition is known, avoiding an extra whole-string encoding/scan on large simple format outputs. The safety predicate is conservative: unknown complex values, unsafe strings, or unsafe literal text do not get the fast-path marker.

References:

• performance optimization #666
• bench/reports/sjsonnet-vs-jrsonnet-gaps.md
• bench/reports/sync-points.md

Result:
large_string_template improves in both Native command orders, JVM JMH does not regress, output equality holds for large_string_template and kube-prometheus, and ./mill --no-server --ticker false --color false -j 1 __.test plus __.checkFormat pass.