fix(backend): JSON-escape record property names in both backends

[xia-chao]

Summary

• JSON-escape statically-known record property names at every key-label site in both backends. JSON.stringify built the label by interpolating the decoded name into a JSON fragment and only ran the C/LLVM string-literal escaper over it, so \ lost its escape, " closed the string and produced invalid JSON, and C0 control bytes were written verbatim.
• The fix encodes the name with JSON.stringify at compile time and keeps the existing literal layer: cStringLiteral/host.cstr then quotes the already-valid JSON text, so the runtime buffer receives valid JSON. Index-signature keys were already correct (they call scr_jb_put_json_str); only the static-label branches were wrong.
• Adds tests/corpus/2853-json-stringify-record-key-escapes.ts, a differential program that covers backslash, double quote, control bytes, optional-field records, index-signature records, a nested record, an array element, the space pretty printer, and a JSON.parse round-trip.

Reported in #312 (that report has the debugger screenshots for both the wrong and the correct path).

Before, on the published 0.1.1:

$ scriptc run repro.ts
{"a\nb":1,"q"q":2,"note":"ok"}
scriptc: TypeError: record has no key (typed slot — no undefined is representable)
scriptc: program killed by SIGABRT

After this change, both backends match Node v24.15.0 byte for byte:

$ scriptc run repro.ts
{"a\\nb":1,"q\"q":2,"note":"ok"}

Validation

• node packages/cli/dist/main.js run tests/corpus/2853-json-stringify-record-key-escapes.ts (LLVM) and --backend c: stdout byte-identical to Node v24.15.0, exit 0
• pnpm vitest run tests/harness/differential.test.ts -t "2853-json-stringify-record-key-escapes" passed
• full tests/harness/differential.test.ts corpus run launched locally; results to follow in a comment
• changed-file ESLint: 0 errors (only pre-existing no-non-null-assertion warnings in the touched file)
• pnpm test:sandbox not run locally (no Vercel Sandbox credentials on this machine)

Follow-ups

The same interpolation pattern is used by the %j format path and island boundary serialization, which share this record writer, so they inherit the fix. No other raw static JSON label site remains: scr_jb_edge_prop only feeds the circular-structure error message (Node prints that name unescaped too), and the index-overflow path already used the runtime escaper.

[vercel]

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[xia-chao]

Please review