fix(types): allow undefined values in populate output

[himanshu748]

populate() accepts undefined input values and preserves them in its result, but DotenvPopulateOutput currently declares every value as string.
Change the output index signature to string | undefined. Add a strict TypeScript regression that rejects assigning an undefined-capable populated value to a required string, plus a runtime test documenting the existing behavior for empty and populated targets with override: true.
This is a declaration correction; the runtime implementation is unchanged. Consumers using strict null checks may need to narrow a populated value before treating it as a string.
Validation recorded in the 12 October 2026 handoff on Linux, Node 24.19.0 and npm 11.9.0:

• The new type test fails against the original declaration with TS2578 and passes with the change.
• The runtime control passes against the original implementation, confirming it already preserves undefined values.
• npm test: build, Standard lint, strict TypeScript checks, and TAP passed; 1,203 passed and 1 skipped.
• npm run test:perf: passed; fast/classic ratio 2.21x for Buffer input and 2.39x for string input, above the 1.5x gate.
  The Windows executable/batch-shim case was skipped on Linux. The full Ubuntu/Windows and Node 20/22/24/26 matrix was not run.
  AI assistance: Codex assisted with this contribution and regression validation.

[motdotla]

appreciate this but this is ai thinking... this is just not a real edge case in the history of .env files. no one uses proto as a key name. not worth the added complexity to support such a non-existent edge case.

[motdotla]

oops that comment above was meant for a different PR. reviewing this one now.

[himanshu748]

oops that comment above was meant for a different PR. reviewing this one now.

Lol🥲will wait.

[motdotla]

merged. thanks! will go out shortly as x.x.8