[Repo Assist] perf: replace O(N2) linear scan in dedupe_packages with O(N) HashMap

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What

Replaces the O(N2) linear scan in dedupe_packages with an O(N) HashMap-based lookup.

Before:

fn dedupe_packages(packages: Vec<Package>) -> Vec<Package> {
    let mut deduped: Vec<Package> = Vec::new();
    for pkg in packages {
        if let Some(existing) = deduped.iter_mut().find(|current| {  // O(N) scan
            current.id == pkg.id && current.source.eq_ignore_ascii_case(&pkg.source)
        }) { ... }
    }
    deduped
}

After:

fn dedupe_packages(packages: Vec<Package>) -> Vec<Package> {
    let mut index: HashMap<(String, String), usize> = HashMap::new();  // O(1) lookup
    let mut deduped: Vec<Package> = Vec::new();
    for pkg in packages {
        let key = (pkg.id.clone(), pkg.source.to_ascii_lowercase());
        match index.get(&key) { ... }
    }
    deduped
}
```

## Why

`dedupe_packages` is called after every `winget list`, `winget search`, and `winget upgrade` result. The previous implementation scanned the accumulating output Vec for each input package, giving O(N2) string comparisons.

| Package count | Old comparisons | New |
|---------------|----------------|-----|
| 200 | ~20,000 | ~200 |
| 400 | ~80,000 | ~400 |
| 500 | ~125,000 | ~500 |

Users with large `winget list` outputs (common on developer machines) will see noticeably faster initial load.

## Behaviour preserved

- First occurrence of a `(id, source)` pair wins in insertion order.
- If a later entry is strictly better (`prefer_package`), it replaces the earlier one in-place, maintaining its position in the result.
- `source` comparison remains case-insensitive: normalised to `to_ascii_lowercase()` for the HashMap key, matching the previous `eq_ignore_ascii_case` guard.

## New tests

- `dedupe_packages_source_comparison_is_case_insensitive` — `"winget"` and `"Winget"` sources are treated as the same
- `dedupe_packages_preserves_insertion_order_of_unique_packages` — three distinct packages emerge in input order

## Test Status

```
cargo check --all-targets  → clean
cargo fmt -- --check       → clean
cargo clippy -- -D warnings → clean
cargo test                 → 232 passed (was 230) — 2 new tests

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