Const suggestion check: step 3 of the correspondences scans every pair for each target #251

Follow-up of #217 and #239 (final review, M3).

In step 3 of the correspondences (src/analysis/ConstSuggestionMatch.cpp), each symbol that proposes a target scans all the pairs already established to find the ones that map onto the same target. With P pairs, a round costs O(P²).

On the self-analysis, the pairs per suggestion reach 256 and the correspondences cost under 50 s summed over the run (48.3 s in the P2 campaign), so the scan is not the main cost today. An index from the target to its sources, kept with the pairs, removes it.

Validation: same pairs and same rounds on the T tests and the corpora.