Implement two-pointer / sliding-window detector (MVP)

[MaximilianRau04]

Context

src/algorithm_pattern_classifier/detectors/ is currently empty. We need a first working detector to validate the detection pipeline end-to-end before adding more patterns.

What

Implement the first concrete BaseDetector for the two-pointer and sliding-window patterns:

• Two-pointer: two index variables initialized at opposite (or same) ends of a sequence, advanced conditionally within a shared loop.
• Sliding-window: a window defined by a start/end index pair that expands/contracts over a single pass, typically tracking a running aggregate (sum, count, max).

Detection should work on the parsed AST (or structural metadata from #1) and produce evidence (line numbers, matched sub-structure) supporting the classification.

Depends on #1.

Acceptance Criteria

• Detector correctly flags reference implementations of two-sum (sorted array, two-pointer) and longest-substring-without-repeat (sliding window)
• Detector does not false-positive on a plain single-pointer linear scan
• Unit tests cover true positives, true negatives, and near-miss cases (e.g. two independent loops that aren't actually two-pointer)