fix(beam): make Emit $N substitution a single left-to-right pass

[dbrattli]

Problem

The Erlang/BEAM printer substituted [<Emit>] placeholders with naive sequential String.Replace($"$%d{i}", ...). Iterating one placeholder at a time over the mutated string has two correctness bugs:

1. $1 corrupts $10 (textual prefix match). Replacing $1 also matches the $1 prefix of $10:

   [<Emit("[$0, $1, $10]")>]
   let f a0 a1 ... a10 = nativeOnly
   f 0 99 ... 1010   // produced [0,99,990] instead of [0,99,1010]

2. A $N sequence inside an already-substituted argument gets re-substituted:

   [<Emit("{$0, $1}")>]
   let pair (a: string) (b: int) = nativeOnly
   pair "$1" 7       // produced {<<"7">>, 7} instead of {<<"$1">>, 7}

Fix

Replace the loop with a single Regex.Replace over \$\d+ — the same approach the JS printer (BabelPrinter.fs) already uses:

let result =
    System.Text.RegularExpressions.Regex.Replace(
        template,
        @"\$\d+",
        fun m ->
            let idx = int (m.Value.Substring(1))
            match Array.tryItem idx printedArgs with
            | Some printed -> printed
            | None -> m.Value   // out-of-range placeholder: leave literal text untouched
    )

This fixes both bugs by construction: \d+ matches the full integer index (so $10 isn't clipped by $1), and Regex.Replace evaluates matches against the original template only, so a $N appearing inside a substituted argument is never re-scanned.

Scope is limited to the Erlang printer (ErlangPrinter.fs); no other backend is touched.

Tests

Added two regression tests in tests/Beam/InteropTests.fs covering both bugs. Full BEAM suite passes (2444 passed, 0 failed).

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