refactor: tsup outExtension migration compatibility

[sxzz]

Summary

• Add deprecated outExtension compatibility as an alias for outExtensions, with runtime mapping and a conflict error when both options are configured.
• Update tsdown-migrate, bundled skills, migration docs, and output format docs for the outExtension -> outExtensions rename and the IIFE/UMD filename behavior.
• Document outputOptions.entryFileNames as the way to preserve custom full filename patterns.
• Add runtime and migration tests for warning, conflict, and output naming behavior.

My recommendation remains that modern libraries should prefer ESM-only output. IIFE/CJS and similar legacy formats are compatibility paths in tsdown, not first-class targets for new library design.

Testing

• pnpm test packages/migrate/src/helpers/tsup-config.test.ts --run
• pnpm test tests/e2e.test.ts --run
• pnpm typecheck
• pnpm lint

closes #949

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commit: e45a242

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds backward-compatible support for the deprecated outExtension option (alias of outExtensions), with conflict detection, migration tooling updates, and documentation clarifying IIFE/UMD filename differences vs. tsup.

Changes:

• Accept outExtension at runtime: warn, map to outExtensions, and throw on conflict.
• Extend tsdown-migrate to rename outExtension → outExtensions for both property and method shorthand syntax.
• Update docs/skills to document the rename and IIFE/UMD filename differences (.iife.js vs .global.js).

Reviewed changes

Copilot reviewed 17 out of 19 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/config/options.ts	Resolve outExtension alias, warn, error on conflict, strip from resolved config.
src/config/types.ts	Add deprecated outExtension field and exclude it from ResolvedConfig.
packages/migrate/src/helpers/tsup-config.ts	Match both pair-key and method-definition identifiers for renames.
packages/migrate/src/helpers/tsup-config.test.ts	Tests for outExtension → outExtensions rename (property + method forms).
packages/migrate/src/helpers/snapshots/tsup-config.test.ts.snap	Snapshots for new rename tests.
tests/e2e.test.ts	E2E tests for deprecation warning and conflict error.
tests/snapshots/deprecated-custom-extension.snap.md	Snapshot for deprecated extension output.
docs/options/output-format.md, docs/zh-CN/options/output-format.md	Note IIFE/UMD default filename and entryFileNames override.
docs/guide/migrate-from-tsup.md, docs/zh-CN/guide/migrate-from-tsup.md	Document outExtension rename and filename differences.
skills/tsdown/references/option-output-format.md	Correct IIFE example output and add filename note.
skills/tsdown/references/option-output-directory.md	Update IIFE filename table and clarification.
skills/tsdown/references/guide-migrate-from-tsup.md	Add rename table and IIFE filename note.
skills/tsdown-migrate/references/guide-option-mappings.md	Add outExtension rename entry.
skills/tsdown-migrate/references/guide-differences-detailed.md	Add rename row and filename differences section.
skills/tsdown-migrate/SKILL.md	Add outExtension to renames table and migration checklist.
snapshots/tsnapi/index.snapshot.d.ts, snapshots/tsnapi/config.snapshot.d.ts	Public type snapshots reflecting new outExtension field and omission.

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