Reuse ValueIndexPair alias in ValueStringHashes

[staging-devin-ai-integration]

Reuse ValueIndexPair in ValueStringHashes alias

Summary

Replaced the duplicated std::pair<std::size_t, std::size_t> type inside ValueStringHashes with the existing ValueIndexPair alias in src/evaluator/include/sourcemeta/blaze/evaluator_value.h.

This keeps the type definition DRY and makes related index-pair usage consistent across evaluator value aliases.

Review & Testing Checklist for Human

• Confirm ValueIndexPair is intentionally the exact semantic/type match for the second element of ValueStringHashes (not just structurally equivalent today).
• Verify the alias change is header-only and does not alter ABI/API expectations beyond readability/maintainability.
• (Optional quick validation) Re-run make configure compile && make locally if you want independent confirmation of formatting/build/test status.

Notes

• Local validation performed: make configure compile and make (passed).
• Link to Devin Session: https://staging.itsdev.in/sessions/c8c93f9f75634842beafe4be8eb36199
• Requested by: bot_apk

[staging-devin-ai-integration]

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