$"{x}" / StringBuilder.Append(x) never returns for a value with an extreme exponent: TryFormat returns false where ToString throws OverflowException #144

What's wrong

For a value whose fixed-point text would be longer than any string can hold (more than Array.MaxLength characters), ToString() checks the required length (PreciseNumber/PreciseNumber.cs:~503-513) and throws OverflowException. TryFormat (PreciseNumber.cs:1354 onwards) has no such check. It just returns false when the text doesn't fit.

ISpanFormattable callers read false as "the destination was too small". DefaultInterpolatedStringHandler (every $"..."), StringBuilder.Append(ISpanFormattable), string.Format and others respond by growing the buffer and calling again. No buffer can ever be large enough, so they grow until an allocation fails or loop indefinitely.

Reproduction

var x = PreciseNumber.Parse("1e-2147483648", CultureInfo.InvariantCulture);
x.ToString();   // OverflowException after ~4 ms
_ = $"{x}";     // still running when killed after a 120 s timeout

Why it matters

The PR #77 fix for #75 deliberately changed TryFormat to return false here instead of throwing. The runaway retry in the BCL's interpolation and StringBuilder paths is a side effect it didn't cover. A single extreme value that reaches a log message or string interpolation now hangs the thread instead of failing fast the way ToString() does. Such a value can come from parsing untrusted input or from repeated division.

Suggested fix

In TryFormat, compute the required length the same way ToString() does:

• If it exceeds Array.MaxLength, throw the same OverflowException. This is a property of the value, not of the destination.
• Return false only when a possible length doesn't fit in destination.

Alternatively, fall back to scientific notation for extreme exponents (option 2 from #75), which makes both paths succeed.

Acceptance criteria

• $"{x}" and new StringBuilder().Append(x) for 1e-2147483648 (and for a very large positive exponent) either throw OverflowException or produce scientific text, promptly, matching ToString().
• A test runs this under a short timeout.
• Existing TryFormat tests still pass, including those that expect false for a genuinely too-small destination.