Create a scalable deterministic node id for indexed code

Problem

The code indexer generates deterministic node IDs by hashing component strings (path, name, etc.) using FxHasher and casting to i64:

fn compute_id(components: &[&str]) -> i64 {
    let mut hasher = FxHasher::default();
    components.hash(&mut hasher);
    hasher.finish() as i64
}

FxHasher produces 64-bit hashes. With the birthday paradox, collision probability reaches 50% at around 4 billion nodes. For large monorepos with millions of definitions across many branches, this becomes a real concern.

The current implementation also affects edges, which reference these node IDs as source_id and target_id.

Proposed Solution

Investigate switching to Int128 for node and edge IDs. This would push the 50% collision threshold to ~10^19 nodes.

Key areas that need changes:

1. ID generation - Update compute_id() to return i128, possibly using a 128-bit hash like xxHash3-128 or SipHash-128
2. Schema types - Add Int128 support to Arrow type mappings and ClickHouse DDL generation
3. Query engine - Update InputNode::node_ids, InputIdRange, and ID filter functions to use i128
4. JSON serialization - JSON doesn't have native Int128; need to serialize as strings to avoid precision loss
5. Result formatting - Add ColumnValue::Int128 variant and handle in response formatting

The JSON serialization is the trickiest part since serde_json will lose precision for values exceeding 2^53. String representation is the safest approach but requires client-side parsing.