Function osToDem

osToDem(os): string
Convert an OS amount (smallest unit) to a DEM string (human-readable).

Always returns a string to preserve precision. The result has exactly one decimal point, with trailing fractional zeros trimmed (but at least one fractional digit retained).

Negative inputs are supported and produce a leading -.
Parameters
- os: bigint
  OS amount as bigint.
Returns string
DEM amount as decimal string.
Example
```
osToDem(1_000_000_000n) // => "1.0"
```
Example
```
osToDem(500_000_000n)   // => "0.5"
```
Example
```
osToDem(1n)             // => "0.000000001"
```
Example
```
osToDem(0n)             // => "0.0"
```
- Defined in src/denomination/conversion.ts:75