fix(ebnf): emit token references for parseable/custom productions so railroad diagrams render

[Pastalikek65]

Fixes #444.

Problem

For grammars using the Parseable interface (e.g. langx's operator-precedence approach), parser.String() produced EBNF referencing undeclared uppercase productions, and cmd/railroad crashed with a nil pointer dereference on the missing production entry:

panic: runtime error: invalid memory address or nil pointer dereference
main.countProductions ... (productions[n.Name].refs++ on a missing key)

Fix

• ebnf.go: *parseable and *custom nodes now emit token references (<custom>) instead of a bare undeclared type name (Custom), consistent with the EBNF invariant (uppercase = production, lowercase = lexer token). These are not real lexer tokens at runtime, but they are the only renderable representation of an externally-parsed production.
• grammar.go: value-receiver implementations of Parseable (e.g. func (Modifier) Parse(...)) panicked with reflect: Elem of invalid type in parseType because t.Elem() was unconditionally called on a non-pointer type. Now handled uniformly.
• cmd/railroad/main.go: nil-guarded productions[n.Name] lookups in both countProductions and generate, so third-party EBNF with unresolved references renders instead of panicking.

Example

Before this PR these EBNF lines would crash cmd/railroad; now they render:

Root = <modifier> Body .
Body = <ident> <custom> .

One test assertion updated (TestParserWithCustomProduction), and one EBNF test added using a value-receiver Parseable (TestEBNF_Parseable) covering both bugs.

Note this is a breaking change to parser.String() output for grammars with parseable/custom productions.

[alecthomas]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 3c50ad3ac2

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Count unresolved references toward production size

When a production consists solely of an unresolved reference, such as Root = Missing ., this branch leaves its computed size at zero. Because mergeSizeThreshold is zero, the post-pass then sets Root.refs to -1, and generate skips the entire production, producing HTML with no diagram instead of the intended unresolved NonTerminal. Increment size for every named term while only guarding the referenced production's refs update.

Useful? React with 👍 / 👎.