Function demToOs

demToOs(dem): bigint
Convert a DEM amount (human-readable) to an OS amount (smallest unit).

Accepts number or string for ergonomics. The result is always a bigint. Strings are preferred for high-precision input because JavaScript number cannot represent every 9-decimal value exactly.

Throws if:
- The input has more than OS_DECIMALS fractional digits.
- The resulting amount is negative.
Parameters
- dem: string | number
  DEM amount as number or decimal string.
Returns bigint
OS amount as bigint.
Example
```
demToOs(1)            // => 1_000_000_000n
```
Example
```
demToOs("0.5")        // => 500_000_000n
```
Example
```
demToOs(100)          // => 100_000_000_000n
```
Example
```
demToOs("1.123456789")// => 1_123_456_789n
```
- Defined in src/denomination/conversion.ts:35