fix: render subscript zero count in default decimalFold format

[NemeZZiZZ]

Problem

The default formatFoldDecimal emits the literal text 0{3} instead of a subscript count:

result[lastIndex] = v.replace(/0*/, `0{${count}}`)   // → "0.0{3}51234"

Verified by execution: formatFoldDecimal('0.00051234', 3) → '0.0{3}51234' — braces printed verbatim on axis labels and tooltips for any high-precision symbol (tokens with pricePrecision 8–10).

The documented sample for this exact feature (docs/@views/api/samples/init-decimalFold-format/index.js) maps the count to Unicode subscript digits — i.e. 0.0₃51234 — so the default implementation contradicts the documented behavior of the option it backs.

Fix

Port the subscript mapping from the docs sample into formatFoldDecimal (module-level subscriptNumbers map, zero-count → subscript).

One deliberate deviation from the sample: the count is subscripted with /\d/g (all digits), not /\d/ (first only) — the sample renders a two-digit count (10+ leading zeros) as 0.0₁2…. With /g it is 0.0₁₂….

Verified outputs: '0.00051234' → '0.0₃51234', '0.000051234' → '0.0₄51234', '1.23456' / '0.0051234' / '0.0000' pass through unchanged.