Runtime: optimize owned string, RegExp, and JSON pipelines

[chrisbbreuer]

Parent: #459
Depends on: #460, #465, #469

Goal

Build owned high-throughput string, RegExp, and JSON pipelines that are competitive with JSC across ASCII, UTF-8, UTF-16, Unicode-heavy, and adversarial inputs.

Scope

• Profile and optimize string representation transitions, flat/slice/rope/concat forms, interning/atomization, hashing, equality, search, replace, split, case conversion, normalization, iteration, and property-key conversion.
• Avoid repeated UTF-8/UTF-16 transcoding while preserving code-unit/code-point semantics, lone surrogates, normalization, locale behavior, and stable embedding views.
• Extend the owned zig-regex integration with reusable compiled plans, literal admission, capture storage reuse, one-pass/DFA/backtrack selection, replacement expansion, Unicode sets/properties, and bounded failure behavior.
• Add a streaming or iterative owned JSON parser/stringifier fast path with exact reviver/replacer/toJSON/property-order/cycle/BigInt/error semantics and bounded nesting.
• Expose tier/runtime/allocator counters and memory profiles for these pipelines.

No-workaround rules

• No system regex/JSON library, ICU, JSC/V8 code, third-party parser, invalid Unicode shortcut, omitted capture/replacer behavior, or input-specific precomputation.
• Cache keys and lifetimes must be bounded, content-correct, realm-safe, and no-GIL synchronized.

Acceptance

• Bench: define the representative dependency-free performance matrix #460 includes small/large, ASCII/Unicode, flat/rope, match/no-match/pathological, parse/stringify, and callback-observable rows.
• test262, Unicode/CLDR generators, regex dependency tests, fuzzing, OOM, deep input, GC, no-GIL, and TSan pass.
• Exact-parent profiles prove lower conversion/allocation/backtracking/hash/dispatch cost with bounded memory.
• Every accepted direct JSC comparison row wins on the declared host with exact output/checksum equality.
• build.zig.zon retains only the owned local zig-regex dependency for regex semantics.

Foundation

Current canonical semantics live in src/value.zig, src/jsstring.zig, src/interpreter.zig, and the sibling ../zig-regex package.