[STiFLeR7]

Summary

• Adds a public RegisterHelper(name, helper) method on Dotprompt (Go) that stores a custom helper for application on the next Compile/Render, matching the JS implementation's defineHelper(name, fn) fluent API (js/src/dotprompt.ts).
• Previously, registering a custom helper required passing it via DotpromptOptions.Helpers at construction time; there was no way to add one afterwards through a dedicated method, and the TODO(#501) comment above RegisterHelpers marked this gap.
• RegisterHelpers (plural, internal pipeline) already applied dp.Helpers and built-ins to a compiled template — this change just adds the missing public single-helper entry point and removes the TODO.

Why

Fixes #501. The Go implementation lacked parity with JS for registering helpers outside of construction-time options, per the issue's acceptance criteria.

Testing

Ran from go/ with Go 1.25 (toolchain pinned by go.mod):

$ go vet ./dotprompt/...
(no output — clean)

$ go test ./... 
ok  	github.com/google/dotprompt/go/dotprompt	15.865s
ok  	github.com/google/dotprompt/go/dotprompt/test	11.443s
?   	github.com/google/dotprompt/go/example	[no test files]

$ gofmt -l dotprompt/dotprompt.go dotprompt/dotprompt_test.go
(no output — clean)

Added TestRegisterHelper covering registration, the fluent-chaining return value, and end-to-end template rendering with the registered helper. All existing tests pass unchanged.

🤖 Generated with Claude Code

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