Replace plugin root regex with SafeDIRootScanner

[dfed]

Summary

• add a lightweight SafeDIRootScanner executable that performs lexical root discovery without SwiftSyntax
• make both build plugins invoke the scanner to produce SafeDIManifest.json plus the explicit per-root output file list before returning buildCommand
• keep SafeDITool's manifest contract unchanged while removing regex-based root discovery and manifest writing from the plugin layer
• make duplicate root file basenames deterministic and collision-safe in the scanner output naming logic
• align SafeDIToolTests with the real plugin path by generating manifests through the scanner while preserving the existing input Swift -> expected output style
• add scanner-focused tests for comment/string/raw-string skipping, deterministic manifest serialization, and duplicate-basename outputs
• add tool regressions for same-file multi-root exact output and duplicate-basename manifest outputs

Why

buildCommand is still required here for incremental performance and compatibility with build systems like Buck and Bazel, which means the plugin has to know its explicit output files up front. #202 introduced the manifest contract and per-root outputs, but its plugin-side root discovery remained regex-driven.

This PR keeps the fast explicit-output path while tightening the weakest part of the stack:

• the plugin now orchestrates instead of guessing
• the scanner owns cheap lexical discovery and manifest generation
• SafeDITool remains the semantic source of truth for validation and code generation

Validation

• swift test
• ./lint.sh

Stack

• base: #202
• this PR stays draft until the full stack is ready

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.92%. Comparing base (f6229a1) to head (8a8c8e1).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           dfed/per-root-output     #203    +/-   ##
======================================================
  Coverage                 99.91%   99.92%            
======================================================
  Files                        37       40     +3     
  Lines                      3501     3838   +337     
======================================================
+ Hits                       3498     3835   +337     
  Misses                        3        3            

Files with missing lines	Coverage Δ
Sources/SafeDIRootScanner/RelativePath.swift	100.00% <100.00%> (ø)
Sources/SafeDIRootScanner/RootScanner.swift	100.00% <100.00%> (ø)
...s/SafeDIRootScanner/SafeDIRootScannerCommand.swift	100.00% <100.00%> (ø)
Sources/SafeDITool/SafeDITool.swift	99.65% <100.00%> (+<0.01%)	⬆️

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