cl: preserve zero values for blank struct fields

[cpunion]

Summary

• emit zero constants for blank fields in static struct initializers
• discard dynamic stores into descendants of blank aggregate fields while preserving function-local right-hand-side side effects
• remove the Darwin/arm64 fixedbugs/issue31546.go run xfail

Review counterexamples

The tests directly cover blank fields, nested blank structs, array descendants reached through IndexAddr, adjacent non-blank fields, and side-effecting function-local initializers. IndexAddr traversal is limited to pointer-to-array storage and has a defensive slice/backing-array boundary counterexample that failed before the review fix. The runtime fixture uses unsafe layout checks instead of reflect so it also builds for embedded targets.

Validation

• go test ./ssa -count=1
• full go test ./cl -count=1 -timeout=35m on the initial implementation (2036.211s)
• final focused cl tests, blankfield IR/runtime fixture, and TestXFailMatch
• focused isBlankFieldStore coverage: 94.1%
• blankfield fixture with host Go 1.24.11, 1.25.7, and 1.26.5
• esp32c3-basic compile/link of the blankfield fixture
• fixedbugs/issue31546.go on Go 1.24.11, 1.25.0, 1.26.0, and 1.26.5
• git diff --check

[fennoai]

Review: preserve zero values for blank struct fields

The change is well-constructed. The isBlankFieldStore walk-up correctly handles the intended cases: a store into a descendant of an aggregate blank field is recognized (the inner non-blank FieldAddr continues up to the blank ancestor and returns true), while a store to a non-blank sibling terminates at default and returns false. Side effects are preserved via _ = p.compileValue(b, v.Val) at the call site, and the static-init path forces blank struct fields to zero. Coverage (littest + internal tests) exercises scalar, nested struct, and array blank fields, plus side-effect evaluation. Removing the fixedbugs/issue31546.go darwin/arm64 xfail is consistent with the fix.

Two non-blocking findings are left inline. No blocking issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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