refactor(beam): give string x and x.ToString() one dispatch

[dbrattli]

Summary

The to-string dispatch was duplicated byte-for-byte between two replacement tables, differing only in whether the type variable was spelled _t or t:

operators, | "ToString", [ arg ]	reached by string x
conversions, | "ToString", _	reached by x.ToString()

Both branch on arg.Type through the same six cases (String, Char, Decimal, the floats, other numerics, Boolean, and a generic fallback) and emit the same code for each.

Why it is worth removing

Nothing marked the two as copies, so a fix applied to one spelling and not the other stays invisible until someone happens to test the neglected one — string x correct and x.ToString() wrong, or the reverse.

#4814 is the near miss. It had to add the same boxed-char peel to both sites, with the same five-line rationale paraphrased twice:

// `box c |> string` arrives as a char under a boxing cast to `obj`, and would otherwise fall
// through to `fable_convert:to_string`, which sees only the integer and prints the codepoint.
let arg = Chars.tryAsChar arg |> Option.defaultValue arg

It got both, but only because the duplication happened to be noticed.

What changed

Extracted to ToString.toStringByType in Beam/Prelude.fs, next to the Chars helpers it depends on, with the peel folded in and its rationale stated once. Both call sites become one line each.

Testing

./build.sh test beam green: .NET 2614 passed / 0 failed, Erlang 2633 PASS / 0 FAIL, entry-point program tests passed.

No new tests, because there is no new behaviour to cover. What is checked instead is that there is none: all 82 generated Erlang files under temp/tests/Beam/src are byte-identical before and after the change (diff -r), so nothing about the emitted code moved.

🤖 Generated with Claude Code