Add the jsx option, with the classic runtime in downlevel mode #1

Adds a jsx option for .tsx input, and pragma and pragmaFrag options for downlevel mode.

transformSync(src, { jsx: true }); // erase mode: JSX kept, every offset preserved
transformSync(src, { jsx: true, mode: "downlevel", pragma: "h", pragmaFrag: "Fragment" });

Behavior #

• jsx defaults to false, so existing callers see no change. With it on, < at an operand start follows TypeScript's .tsx rule. It is JSX unless it starts a generic arrow (<T,>, <T extends U>, <T = U>).
• Erase mode keeps the JSX and erases types inside it. It also blanks type arguments on tags (<Select<Option> />). This is a deliberate difference from ts-blank-space, which keeps them and so produces output that no JSX tool accepts.
• Downlevel mode writes the classic runtime, factory(tag, props | null, ...children), as ts.transpileModule does with jsx: "react". Every line is preserved. A leading /** @jsx h */ or /** @jsxFrag F */ comment overrides the options, following TypeScript's pragma rules.

docs/modes.md has the full rules.

Engine #

• lexChild() lexes JSX children with no trivia skipping. When a child token runs out of input, push() re-lexes it the same way on the next chunk.
• Each element is one S_JSX frame in strip.zena. The parser is in both builds.
• The classic-runtime emitter is engine/jsx.zena. It is linked into downlevel.wasm only, through the hook seam.
• engine/jsx-entities.wat is TypeScript's HTML entity table, generated from its own map.
• ABI: bit 1 of init's argument turns on JSX. A new pragma(len, fragLen) export passes the factory names.

Cost #

	main	this PR
erase.wasm gzip	12,622 B	13,648 B
downlevel.wasm gzip	17,838 B	22,205 B
wrapper gzip	2,595 B	2,852 B

Throughput is unchanged with jsx off: about 172 MB/s in erase mode and 189 MB/s in downlevel mode on both builds. The erase engine pays for the parser even with jsx off. A separate JSX-capable erase engine would avoid that, but it is not in this PR.

Verification #

• New judge, test/jsx.mjs (pnpm test:jsx, added to pnpm test):
  • Erase mode is compared with ts-blank-space on TSX, with the type-argument difference applied.
  • Downlevel mode must keep the line count and match the canonical AST of transpileModule on ts-blank-space's output.
  • Chunked input must give the whole-input output in both modes.
  • A --self-test proves the judge rejects broken output.
• corpus/jsx.jsonl: 51/51.
• pnpm test:jsx:typescript, every .tsx in TypeScript 6's compiler and conformance suites: 333/333 (62 skipped because they don't parse cleanly as TSX).
• Existing suites unchanged: pnpm test, and pnpm test:erase:typescript at 11,736/11,736.

Open questions #

• Should @jsxImportSource and @jsxRuntime be reported? They are currently ignored and written as the classic runtime, but TypeScript switches those files to the automatic runtime.
• JSX in output that downlevel mode copies verbatim, such as a parameter decorator's argument, stays untransformed. No corpus has hit this.