Add writeIniFile() for song.ini emission

[elicwhite]

Context

First of the upstream writer stack: adds `song.ini` serialization.

```ts
export function writeIniFile(metadata: IniMetadata): string
export type IniMetadata = Partial & {
extraIniFields?: { [key: string]: string }
}
```

Emission rules

• `[song]` header
• Known fields emitted in the canonical `defaultMetadata` order (derived from `Object.keys(defaultMetadata)` — no hardcoded field list, so new fields added to `defaultMetadata` automatically participate in writing)
• Fields whose value is `undefined` are skipped
• Booleans format as `True`/`False` (Clone Hero convention)
• `extraIniFields` appended after known fields, in insertion order
• CRLF line endings

Round-trips cleanly through `scanIni`.

Depends on

PRs #97 + #98 + #99 — stacked above them.