feat: stop scanning and keep partial results

[ShivamB25]

Summary

• make Ctrl+C during an interactive scan stop scheduling new work instead of quitting
• keep the partial directory tree and transition directly to navigable results
• preserve point-in-time preview safety while analyzers are still mutating the live tree
• apply cooperative cancellation consistently to parallel, stable, sequential, stored, SQLite, and top-directory analyzers
• document the Ctrl+C behavior

Behavior

• first Ctrl+C during a scan requests cancellation and shows a stopping state
• in-flight filesystem operations finish safely; no new recursive work is scheduled
• the resulting partial tree remains available for navigation
• later Ctrl+C follows the normal quit path
• cancellation state resets before a subsequent scan

Verification

• go vet ./...
• go test ./...
• go test -race ./pkg/analyze ./tui
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.2 run ./...
• interactive smoke: scan /, send SIGINT while active, observe Stopping scan and keeping results..., then reach a populated results screen

Closes #150

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.93%. Comparing base (fe605ec) to head (9c4e60a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #606      +/-   ##
==========================================
+ Coverage   85.89%   86.93%   +1.04%     
==========================================
  Files          55       55              
  Lines        6329     6521     +192     
==========================================
+ Hits         5436     5669     +233     
+ Misses        689      657      -32     
+ Partials      204      195       -9     

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