perf: batch individual file inputs for parallel formatting

[ansemb]

Problem

When passing 200+ individual file paths to CSharpier (e.g. with pre-commit hooks), formatting is significantly slower than passing a directory containing the same files.

Note: this was run on Mac M1. Showing the ms is only meant as an indication of large difference between inputting a folder and multiple files.

Directory:

dotnet csharpier format path/to/project/dir
Formatted 567 files in 928ms.

Files:

dotnet csharpier format \
  path/to/project/dir/file1 \
  path/to/project/dir/sub/file2 \
  ...

Before (293 files, individual paths): Formatted 293 files in 40886ms.
After (293 files, individual paths): Formatted 293 files in 859ms.

Root Cause

In CommandLineFormatter.FormatPhysicalFiles, each individual file path was processed inside the input loop with:

1. A new OptionsProvider per file (expensive — reads .csharpierrc, .editorconfig, .gitignore from disk and walks up directories)
2. A new FormattingCache per file (reads/writes a separate cache JSON each time)
3. Sequential formatting (no parallelism)

In contrast, directory inputs already used a single OptionsProvider, a single FormattingCache, and parallel formatting via Task.WhenAll.

Solution

Individual file paths are now collected into a pending list during the input loop. After all inputs are processed:

1. A common ancestor directory is computed from all file paths using a new GetCommonAncestor helper
2. A single OptionsProvider is created, pre-seeded at the common ancestor. Config resolution still walks up from each file's own directory via its internal ConcurrentDictionary caches, so per-directory configs (e.g. different .csharpierrc in different subdirectories) are respected correctly.
3. A single FormattingCache is shared across all files
4. All files are formatted in parallel via Task.WhenAll, matching the directory code path

Directory inputs continue to work exactly as before.

Tests Added

• Multiple_Files_Should_Be_Formatted — verifies files from different subdirectories are formatted
• Multiple_Files_Should_Respect_Per_Directory_Config — verifies per-directory .csharpierrc is respected when using a shared OptionsProvider
• Mixed_Files_And_Directory_Should_Format_All — verifies mixed directory + individual file inputs
• GetCommonAncestor_Returns_Common_Path — 5 parameterized cases covering same dir, nested, divergent, root-only, and empty-string fallback