eval: report validation error locations

[raphael]

Summary

• Add file:line prefixes to validation errors when the failing expression has a DSL function pointer, making it straightforward to locate the offending design declaration.
• Refactor computeErrorLocation to use runtime.CallersFrames instead of repeated runtime.Caller calls, which is robust to inlining and avoids hardcoding call-depth assumptions.
• Add isGoaSourceFile to reliably skip internal Goa frames even when inlining collapses the stack.
• Add validationErrorLocation / dslFuncLocation to derive locations from the expression's DSL function pointer for Validate()-phase errors.
• Add tests covering DSL errors, validation errors with and without locations, and cross-package RunDSL error propagation.

Test plan

• make test — all passing
• make lint — 0 issues
• New tests in eval/error_location_test.go and eval/run_dsl_error_location_test.go
• Existing expr/ validation tests updated for new location prefixes