fix(controller): preserve concurrent output order

[ekalinin]

Summary

• process files through a shared worker pool in serial and parallel modes
• cap parallel processing at eight concurrent files
• collect indexed results and print TOCs in CLI argument order
• finish queued jobs safely after errors or context cancellation

Verification

• env GOTOOLCHAIN=go1.21.13 go test ./...
• env GOTOOLCHAIN=go1.26.5 go test -race ./...
• go test -count=50 ./internal/controller
• go vet ./...
• golangci-lint run ./...
• go mod tidy -diff
• go mod verify

[codecov]

Codecov Report

❌ Patch coverage is 92.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.47%. Comparing base (590ee6d) to head (3ff0ed8).

Files with missing lines	Patch %	Lines
internal/controller/file.go	92.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   79.31%   80.47%   +1.16%     
==========================================
  Files          28       28              
  Lines         701      712      +11     
==========================================
+ Hits          556      573      +17     
+ Misses        101       96       -5     
+ Partials       44       43       -1     

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