feat: pre-allocate Keys/Values/Map slices using Len

[millerjp]

Summary

Keys() and Values() start with nil and grow via append — bench.txt shows ~11 allocs/op at 1000 entries from slice regrowth alone. Map() similarly uses make(map[K]V) with no capacity. Pre-sizing with Len() halves those allocations at the cost of one extra O(n) traversal — a worthwhile trade-off, but only if documented so callers can make an informed choice.

Scope

• Keys(): keys := make([]K, 0, m.Len())
• Values(): values := make([]V, 0, m.Len())
• Map(): out := make(map[K]V, m.Len())
• Godoc on each method notes that Len() is called internally, doubling the traversal cost.

Acceptance criteria

1. Benchmark numbers drop to ≤ 2 allocs/op for each method at 1000 entries.
2. bench.txt regenerated and committed.
3. make bench-regression green.
4. Godoc documents the double-traversal trade-off.

Source: code-reviewer + performance-reviewer.