Fix prehash cache bypass causing full hash on all files in second scan

[darktorres]

Summary

• Cached prehash entries were appended to pre_checked_map unconditionally, skipping the len() > 1 collision filter that freshly-computed entries go through. On a second scan where all files are cached, every file was forwarded to full hashing regardless of whether their prehashes actually collided.
• Also fixed a latent bug where a cached file and a freshly-computed file sharing the same prehash were never detected as a collision, since each loop only checked within its own set.
• Both loops are now replaced with a single merge-then-filter pass over a combined (size -> hash -> files) map.

Test plan

• Ran cargo check -p czkawka_core — clean compile
• Ran existing duplicate tests (cargo test -p czkawka_core -- duplicate) — all pass
• Validated in the app: second scan no longer full-hashes all files

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