Scan a file for its function and constant declarations

[Firehed]

Slice S3.7b (#390). DeclarationScanner reads the functions and constants a parsed
file declares, so S3.7d can derive a name→file index for the two symbol namespaces
Composer cannot address by name.

• Qualified names come from namespacedName (set by NameResolver during
  ParserService::parse()), not hand-tracked Stmt\Namespace_ — the shape SC.3 is
  removing elsewhere.
• Covered: namespaced and global declarations, every declarator of a multi-name const,
  literal define() in any casing, and declarations nested in a conditional (the polyfill
  shape, which a top-level-only walk would drop).
• Out of scope, per Plan 0002 §3's locate-only limitation: a computed define() name, and
  anything reached only through a require/include.

QualifiedName lands here rather than in S3.7c as the manifest said — the scanner is its
first caller. Manifest corrected in this branch; fromClassName() is left for S3.7c,
where FilesystemBackend needs it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.59%. Comparing base (12a702a) to head (b68b848).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #393      +/-   ##
============================================
+ Coverage     98.58%   98.59%   +0.01%     
- Complexity     1758     1774      +16     
============================================
  Files           116      119       +3     
  Lines          4451     4488      +37     
============================================
+ Hits           4388     4425      +37     
  Misses           63       63              

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