feat: add genExportDefault for ESM export default support

[MFA-G]

Closes #77

Problem

knitwork can generate import, export, and dynamic import() statements, but there is no helper for generating export default <expression>.

Changes

Adds genExportDefault(expression) which returns an export default <expression>; statement.

The expression is used as-is (raw, not escaped or quoted), consistent with knitwork's existing raw-expression helpers such as genObjectFromRaw. Serialization of runtime values is left to the caller, e.g. genObjectFromValues(...) or JSON.stringify(...). This keeps the API unambiguous and composable rather than guessing whether a string is a value or an expression.

genExportDefault("foo");
// ~> export default foo;

genExportDefault("() => {}");
// ~> export default () => {};

genExportDefault(genObjectFromValues({ foo: "bar" }));
// ~> export default { foo: "bar" };

• src/esm.ts: implement and document genExportDefault
• test/esm.test.ts: add cases for identifiers, objects, arrow functions, strings, and numbers
• README.md: regenerated via automd from the new JSDoc

Validation

• pnpm vitest run — 64 tests passing (5 new)
• pnpm lint — eslint + prettier clean
• pnpm build — unbuild succeeds, genExportDefault exported in both ESM and CJS

Summary by CodeRabbit

• New Features

  • Added a new helper to generate ESM export default statements from a provided expression.
  • The helper supports a range of valid JavaScript expressions, including objects, functions, strings, and numbers.

• Documentation

  • Expanded the README with clearer usage guidance and examples showing the generated output format.
  • Added a note explaining that expressions are inserted directly, along with a recommendation to use serialization helpers when needed.

• Tests

  • Added coverage for multiple expression types to verify the generated output matches expectations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 50db1ab0-11f5-4385-8e2a-14c2a76ddc7b

📥 Commits

Reviewing files that changed from the base of the PR and between a95f4d4 and b45dcb4.

📒 Files selected for processing (3)

• README.md
• src/esm.ts
• test/esm.test.ts

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📝 Walkthrough

Walkthrough

Adds genExportDefault(expression) to generate export default ...; strings, documents its direct expression interpolation, and adds tests covering several expression forms.

Changes

Export default helper

Layer / File(s)	Summary
Helper and docs src/esm.ts, README.md	genExportDefault(expression) is added to src/esm.ts, and README.md documents the direct interpolation behavior with example outputs.
Helper tests test/esm.test.ts	genExportDefault is imported and tested against identifier, object literal, arrow function, string literal, and numeric literal inputs.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

I’m a bunny with a fluffy grin,
I hop to the code and let exports in.
default now dances, neat and bright,
With tests that hop along just right.
🐰✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: adding genExportDefault for ESM default exports.
Linked Issues check	✅ Passed	The PR implements support for generating export default <exports>; via a new helper and tests it, matching issue #77.
Out of Scope Changes check	✅ Passed	The changes are limited to the new helper, its tests, and related README docs, with no obvious unrelated additions.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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