Normalize undefined-identifier eval error message

[posthog]

Problem

• The eval team loses visibility: one class of user mistake mints a fresh error tracking issue every time, so a real regression in this code path is easy to miss.
• When circuit code references a part number that was never imported (A_12401610E4_2A, TXB0104RGYR), the engine throws a ReferenceError and the catch at lib/eval/import-local-file.ts splices the identifier straight into the thrown message.
• Part numbers are unique per project, so every project that makes the same mistake gets its own fingerprint and its own issue.
• The browser splits it again: WebKit says Can't find variable: X while V8 and Gecko say X is not defined.
• Nothing is broken for users — the eval runner catches genuine user-code errors correctly (mechanism.handled is true). The cost is observability noise, and it grows with every new project.

Changes

• Normalize the message before it leaves the catch: keep a stable prefix and move the identifier into a trailing suffix.
• Match both engine wordings so the WebKit and V8/Gecko variants land together.

- Error evaluating "entrypoint.tsx": A_12401610E4_2A is not defined
- Error evaluating "entrypoint.tsx": Can't find variable: TXB0104RGYR
+ Error evaluating "entrypoint.tsx": undefined identifier "A_12401610E4_2A"
+ Error evaluating "entrypoint.tsx": undefined identifier "TXB0104RGYR"

• The identifier stays visible in the suffix for the person debugging.
• Other eval-wrapper errors (for example "not exported by") keep their exact message.
• Add tests/examples/example10-undefined-identifier.test.tsx.

Agent context

• The wording changed once before in fe347fa, so both old and new phrasings appear in the data today. This change targets only the undefined-identifier family.
• example09-not-defined-component.test.tsx asserts a "not exported by" message, which this change leaves unchanged; that test still passes.

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