fix(eval): stabilize eval error fingerprints and accurate .js type diagnostic

[posthog]

Summary

The eval engine surfaced confusing "…" is not exported by "…" errors for .js imports and filed a fresh error-tracking issue for each occurrence of the identical error. Two tightly related changes in the same area:

• Stable fingerprints. Errors thrown from the dynamically generated Function() body in evalCompiledJs carried anonymous/eval stack frames whose line/column offsets shift with the size of the compiled code, so error tracking fingerprinted otherwise-identical errors as separate issues. evalCompiledJs now strips those generated frames, leaving a stable stack anchored at real source locations so repeat occurrences collapse into one issue.
• Accurate .js type diagnostic. The .js/.mjs branch of import-local-file now runs getTypeExportsFromCode and sets __typeOnlyExports__, mirroring the .ts/.tsx branch, so type-only exports surfaced through .js modules get the accurate "this is a type, use export type" diagnostic instead of the generic message.

Why

One logical eval failure was fragmenting the error-tracking inbox into multiple issues (the identical defaultProps/./external.js message spawned three distinct issues within seconds), and .js imports skipped the friendlier type-only diagnostic that the .ts branch already provided. Measured impact is small — this is primarily inbox-noise cleanup.

Notes for reviewers

• The type-only-export path is reachable when a .js module re-exports a type from a .ts module (a literal export type inside a .js file is rejected by the transpiler before it ever runs). The getTypeExportsFromCode addition brings the .js branch to full parity with the .ts branch for that case, and is harmless otherwise — the proxy only consults __typeOnlyExports__ for names that are genuinely absent at runtime.
• The frame-stripping regex deliberately targets only generated-function frames (<anonymous>:, eval at …, bun's bare at anonymous () and leaves legitimate anonymous library frames intact.

Tests

• tests/features/eval-error-stable-stack.test.ts — asserts eval errors carry no generated frames and that identical errors produce identical fingerprint-relevant frames regardless of compiled-code size.
• tests/features/type-import-check-js.test.tsx — asserts the accurate "is a type" diagnostic for a type imported through a .js module.

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Created with PostHog Code from an inbox report.

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