perf(scanner): speed up root-kind lookup on emit path

[ajayk]

Summary

newRootKindLookup returns a closure invoked once per emitted record. The previous implementation, on every call:

1. Ran filepath.Abs even when the input was already absolute.
2. Built r.Path + string(filepath.Separator) inside the inner loop for each root.
3. Did a full linear scan over all roots, tracking the longest match with bestLen.

This PR pre-computes the path + separator prefix once per root at construction time, sorts roots longest-first (stable, so equal-length ties match the prior strict-greater tie-break), and returns on the first prefix hit. The inner closure also skips filepath.Abs when the input is already absolute.

Benchmark (Apple M2 Pro, 64 lookups per op, 6 configured roots)

	ns/op	allocs/op
before	~11,700	0
after	~3,540	0

~3.3x faster on the per-record emit path. Allocations were already 0 in both (escape analysis eats the inline concat), so the win is pure CPU from avoiding repeated string building and linear scans after the longest match.

Test plan

• New TestNewRootKindLookup covers exact match, deep child match, longest-match tie-break, outside-all-roots, and the prefix-overlap-without-separator edge case.
• go test ./... — all packages pass.
• BenchmarkNewRootKindLookup added for future regression tracking.