bug: Date values in WHERE conditions serialized as JS toString() instead of SQL literal

[adrianbrowning]

Symptoms

When using a Date value in a .where() condition, the generated SQL contains the JS Date.toString() output instead of a quoted SQL timestamp literal.

Example:
```ts
.where({ 'timestamp': { op: '>', value: twoDaysAgo } })
```

Generated SQL:
```sql
WHERE `timestamp` > Tue Mar 10 2026 13:16:12 GMT+0000 (Greenwich Mean Time)
```

Expected SQL:
```sql
WHERE `timestamp` > '2026-03-10T13:16:12.000Z'
```

Root Cause

In src/extend.ts, applyCondition() only checks typeof value === 'string' when deciding whether to quote/escape a value. Date objects fall through to the non-string branch and are inserted directly into template literals, triggering implicit JS .toString() serialization.

Affected paths in applyCondition():

• Line ~444: op comparison (>, >=, <, <=, !=, =)
• Line ~430: BETWEEN start/end values
• Line ~425: IN/NOT IN value arrays
• Line ~459: bare equality fallback

Affected Types

DateCondValue in the type system correctly accepts Date objects — the type support exists but runtime serialization is missing.

```ts
type DateCondValue =
| Date
| { op: COND_NUMERIC_OP; value: Date }
| { op: 'BETWEEN'; values: [Date, Date] }
| { op: 'IN' | 'NOT IN'; values: Array };
```

Proposed Fix

Add a Date check alongside the string check in applyCondition() — convert to ISO string and wrap in SQL quotes:

```ts
const sqlVal = (v: unknown) =>
typeof v === 'string' ? `'${esc(v)}'`
: v instanceof Date ? `'${v.toISOString()}'`
: v;
```

Apply this helper consistently across all value-insertion points in applyCondition().