Report each builtin's own name when it errors - #1008

Summary

Two builtins in the standard library are registered under a different function's name, so when one of them fails, ytt names a function the user never called.

pkg/yttlibrary/math.go:118 registers the mod member as m.newBinaryBuiltin("round", math.Mod). core.ErrWrapper prefixes every error with f.Name() (pkg/template/core/errs.go:30), so math.mod(5) fails with round: expected exactly two arguments. math.round is a real function in the same module, so the message points at the wrong call.

pkg/yttlibrary/overlay/api.go:323 returns the matcher built by overlay.not_op as starlark.NewBuiltin("overlay.or_op", ...), copied from OrOp directly above it. An error raised while matching under @overlay/match by=overlay.not_op(...) comes back as overlay.or_op.

Each builtin now carries its own name. Nothing else changes: mod still calls math.Mod and not_op still negates its matcher.

I checked every other starlark.NewBuiltin call under pkg/ for the same slip. These two are the only ones that name a different function.

Testing

One filetest per module, both asserting the error text:

• pkg/yamltemplate/filetests/ytt-library/math-mod-err.tpltest
• pkg/yamltemplate/filetests/ytt-library/overlay/not-op-err.tpltest

With the two source changes reverted and the new tests kept, go test -count=1 ./pkg/yamltemplate/... fails on both, showing the old names:

--- FAIL: TestYAMLTemplate/filetests/ytt-library/math-mod-err.tpltest
        result:   - round: expected exactly two arguments
        expected: - mod: expected exactly two arguments
--- FAIL: TestYAMLTemplate/filetests/ytt-library/overlay/not-op-err.tpltest
        result:   - overlay.apply: Array item on line stdin:13: overlay.or_op: expected starlark.Bool, but was starlark.String
        expected: - overlay.apply: Array item on line stdin:13: overlay.not_op: expected starlark.Bool, but was starlark.String

Local runs on macOS arm64 with Go 1.26.4, on 7 Sep 2026:

• go test -count=1 ./... passes, test/e2e and test/filetests included, against a ytt binary built from this branch
• gofmt -l pkg/yttlibrary/math.go pkg/yttlibrary/overlay/api.go prints nothing
• git diff --check is clean

Branched from develop at 6a94bf4.