Dedupe the hand-rolled file:// conversion behind FileUri

[Firehed]

Slice SC.4: file:// URI and path conversion was hand-rolled in three live places,
each handling the scheme slightly differently and none of them consistently handling
percent-encoding. One FileUri replaces them.

Pre-existing duplication owned by no plan step, which is what the SC.* block is for. It
surfaced while splitting S3.7, whose locator wanted a fourth copy; S3.7c is gated on this
landing rather than adding one.

The three sites

Site	Before	After
FilesystemBackend	private pathFromUri (decoded) + 'file://' . $path	FileUri::toPath / FileUri::fromPath
DefaultClassInfoFactory	private uriToPath — stripped the scheme, did not decode	FileUri::toPath
Location::fromFileLine	str_starts_with(...) ? $file : 'file://' . $file — never encoded	normalized through the seam

Two behavior fixes fall out

Both were latent: they only appear when the workspace sits under a path containing a
reserved character, which is ordinary on macOS (~/My Projects/...).

• DefaultClassInfoFactory did not decode, so a class in a directory with a space
  produced a path that does not exist on disk.
• Location::fromFileLine did not encode, and its output is what a definition or hover
  response carries to the client — an unencoded space or # makes that an invalid
  URI. Covered by a test committed red before the fix.

Not in scope

WorkspaceIndexer:61 holds a fourth copy and is deliberately untouched: it is dead code
with zero references, and slice SC.1 deletes the file outright.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (dc647dc) to head (a594d54).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #391      +/-   ##
============================================
+ Coverage     98.55%   98.57%   +0.02%     
+ Complexity     1753     1751       -2     
============================================
  Files           115      116       +1     
  Lines          4434     4432       -2     
============================================
- Hits           4370     4369       -1     
+ Misses           64       63       -1     

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