fix: preserve underscores in formula struct names

[MeteorsLiu]

Summary

• derive the generated Formula struct name by trimming the full _llar.gox suffix
• preserve underscores in names such as cpu_features
• add a regression fixture and correct the classfile guidance

Testing

• go test -ldflags="-checklinkname=0" ./internal/formula -count=1
• go test -ldflags="-checklinkname=0" ./internal/modules -count=1
• full ./... reaches the existing TestE2E_RealLibpngBuild Clang 21 failure, reproduced unchanged on goplus/main

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[fennoai]

Review: preserve underscores in classfile struct name

The core change is correct and well-targeted. Switching from strings.Cut(filename, "_") to strings.TrimSuffix(filepath.Base(path), "_llar.gox") properly fixes the truncation of names like cpu_features -> cpu, matches xgo's own classfile naming convention, and using filepath.Base additionally handles paths with directory components. The doc comment, AGENTS.md, and a targeted regression test with testdata were added in sync.

On the removed validation (formula.go:125): dropping the old !ok error for filenames without _ is not a functional regression. If a name lacks the _llar.gox suffix, TrimSuffix returns it unchanged and the following interp.GetType (line 128) fails closed with a clear "struct name not found" error. The production caller (internal/modules/source.go:109) also enforces HasSuffix(path, defaultFormulaSuffix) before loading, so the missing-suffix case is effectively unreachable. Security and performance passes found no issues.

Follow-up (out of this PR's scope)

internal/modules/comparator.go:55 still derives its struct name with strings.Cut(filepath.Base(path), "_") — the exact pattern this PR replaced. A _cmp.gox file with an underscore in its name (e.g. cpu_features_cmp.gox) would resolve to struct cpu and fail GetType. Worth a follow-up applying the same TrimSuffix(..., "_cmp.gox") fix there.

Optional

• Consider a small subtest for a valid, parseable file lacking the _llar.gox suffix to lock in the "GetType safety net" error behavior this change now relies on.
• The _llar.gox literal is now hard-coded in several places (formula.go:125, plus defaultFormulaSuffix in source.go:23 and the Ext registration in ixgo/classfile.go:31). Sharing one constant would prevent drift.

[fennoai]

[P3] Doc snippet drops filepath.Base(), now inconsistent with code

This quotes strings.TrimSuffix(filename, "_llar.gox"), but the actual code is strings.TrimSuffix(filepath.Base(path), "_llar.gox") (formula.go:125). The filepath.Base step is load-bearing: path can carry directory components, so the snippet as written wouldn't handle a nested path. Either match the code exactly or keep it conceptual ("the filename without the suffix") as line 60 does.

[luoliwoshang]

LGTM