Extend zero-copy string slice views to bounded-waste substrings

[lahma]

What

String.prototype.substring(start, end) clamps a negative end to 0 and then swaps so the larger index is the end. So str.substring(12000, -1) on a 131072-char string yields a 12000-char result whose pinned backing-string remainder (119072 chars) exceeded JsString.CreateSliced's zero-copy retention guard (length*2 >= source.Length → 24000 < 131072), forcing a full string.Substring copy on every call. The visually-identical str.slice(12000, -1) resolves -1 relative to length without swapping → a 119071-char span → passes the guard → zero-copy view. That asymmetry is why slice was ~1% of dromaeo-object-string's allocation while substring was 89.85% (116 MB/iter).

This adds an absolute-waste cap to the guard: a slice also becomes a SlicedString view when the bytes it pins but never uses (source.Length - length) stay within SliceViewMaxWastedChars (128K chars). So a moderate slice of a large source avoids the per-call copy while retention stays bounded (≤ result + 256 KB). Small slices still copy.

The PR also corrects StringSliceBenchmarks.SubstringLargeDiscard, which used substring(12000, str.length - 1) (positive args, no swap → already a view) and so never measured the dromaeo shape that copies — the reason #2506's benchmark reported substring as a small fraction.

Benchmarks (default job, stash A/B)

Benchmark	Baseline	This PR	Δ
SubstringLargeDiscard — Allocated	117,776 KB	550 KB	−99.5%
SubstringLargeDiscard — time	6.39 ms	1.96 ms	−69%
SliceSmall (must still copy)	628.57 KB	628.57 KB	flat
SliceThenRead / SliceLargeDiscard	—	—	flat

End-to-end dromaeo-object-string allocation 129 MB → 14.6 MB (−88.7%) (substring 116 MB → 1.6 MB), via MemoryProbe. Jint stays rank #1 on object-string by time.

Tests

• Jint.Tests net10 (3067) + net472 (3005)
• Jint.Tests.PublicInterface net10 (79) + net472 (79)
• Test262: 99260 passed, 0 failed (substring/substr/slice suites + the indexOf/startsWith/split/equality consumers that operate on the returned view)

🤖 Generated with Claude Code