perf: use unsynchronized StringBuilderWriter in TomlRenderer

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Motivation

std.manifestTomlEx had three sources of avoidable overhead on the hot manifestation path:

1. Synchronized writer. TomlRenderer and ManifestModule.evalRhs rendered into a java.io.StringWriter, whose backing StringBuffer pays a monitor enter/exit on every write/flush. The FastMaterializeJsonRenderer already uses the unsynchronized StringBuilderWriter (perf: speed up manifest JSON rendering #874); TOML did not.
2. Redundant field lookups in renderTableInternal. Each key's Val.Obj.value(k) was resolved twice — once to classify scalar vs section, then again to render or recurse. The cache deduplicates the result, but the lookup itself still costs.
3. Wasted indexing work. visibleKeyNames was iterated and each key binary-searched back into sortedVisibleKeyNames — sortedVisibleKeyNames can be iterated directly, skipping O(n log n) compares per table.

Modification

Two commits:

• perf: use unsynchronized StringBuilderWriter in TomlRenderer — Swap TomlRenderer and the manifestTomlEx render path in ManifestModule from java.io.StringWriter to the package-private StringBuilderWriter. std.deepJoin keeps StringWriter (separate concern).
• perf: cache resolved field values and skip binary search in renderTableInternal — Resolve each field once into a resolved: Array[Val] during section classification and reuse it during render/recurse; iterate sortedVisibleKeyNames directly (removes the now-unused sortedKeyIndex binary search); hoist childIndent = cumulatedIndent + indent out of the section loop (was an identical allocation per sibling section); pre-size the output StringBuilderWriter to 1 KiB so small/medium outputs skip the first ~6 doublings.

Output is byte-identical (verified at 1,228,186 bytes on the benchmark workload).

Result

Scala Native, hyperfine A/B against master (fc292fa6). Workload: object comprehension over 8000 small tables → ~1.2 MB TOML output (render-dominated). Four interleaved-order passes, --warmup 10 --min-runs 100 --shell=none:

pass	order	before mean	after mean	before min	after min	min ratio
1	before → after	59.4 ± 2.7 ms	53.2 ± 23.4 ms	55.4 ms	43.8 ms	1.27×
2	after → before	64.1 ± 7.7 ms	51.8 ± 12.2 ms	56.4 ms	43.7 ms	1.29×
3	before → after	64.1 ± 8.1 ms	53.2 ± 14.3 ms	56.4 ms	42.0 ms	1.34×
4	after → before	63.3 ± 14.3 ms	49.2 ± 3.7 ms	57.2 ms	42.8 ms	1.34×

Mean is noisy on the host (1.12× – 1.29×), but after is faster in every one of the 4 passes and the min values are tight at ~1.27–1.34× faster (best observed: 42.0 ms vs 56.4 ms, ~25.5% reduction). Output byte-identical, 1,228,186 bytes both sides.

For comparison, the StringBuilderWriter swap alone (commit 1) measures ~1.08–1.14× min; the cache + binary-search elimination + childIndent hoist (commit 2) lifts that to ~1.27–1.34× min.

Test plan

• ./mill __.reformat
• ./mill 'sjsonnet.jvm[3.3.7]'.test — 519/519 pass
• Scala Native A/B hyperfine — 4 interleaved-order passes, all positive; output byte-identical

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Rebased onto current master (fc292fa6). The companion commit "speed up manifest JSON rendering" was merged separately as #879, so this PR now contains only the TomlRenderer / ManifestModule changes.

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@stephenamar-db rebased