Split S3.7 and re-scope Step 3b into reviewable slices

[Firehed]

Re-scopes the remaining Step 3b slices so each is reviewable as one unit, and records
two decisions found while sizing them.

Why

S3.7 was built as a single slice (#388: 622 src lines over 17 files, 30 commits) and is
too large to review as one unit. The seam it missed is already in the code: a class-like
lookup is arithmetic on the name (findFile — five lines, the old ComposerClassLocator
verbatim), while a function or constant lookup has no name→file map and must derive one
by parsing the autoload.files set. Those are two different reviews.

Sizing the rest of Step 3b against the merged slices (S3.2 +58/−40, S3.4 +171/−28,
S3.1 +305/−47) showed S3.8 and S3.9 heading the same way.

What changes

• S3.7 → S3.7a–d. autoload.files into the autoload map; the per-file declaration
  scan; the kind-agnostic SymbolLocator with its class-like branch (behavior-preserving,
  proven by the existing golden); the derived function/constant index (new behavior, new
  fixtures). S3.7a and S3.7b are independent inputs to S3.7d.
• S3.8 → S3.8a–c, cut by symbol namespace rather than by layer. A layer cut would land
  SymbolBackend methods no backend implements. S3.8c carries the SymbolResolver edit,
  so S4.2 now serializes against it rather than against S3.8 as a whole.
• S3.9 → S3.9a–b, cut on provability. S3.9a widens searchClassLikes to a kind
  parameter with every Step P golden frozen; S3.9b makes the backends answer function
  search and rewrites only the function-surface golden.
• SC.4 added. file:// conversion is hand-rolled in four live places, each differing
  in how it handles the scheme and percent-encoding. Pre-existing duplication owned by no
  step, which is what the SC.* block is for. S3.7c is gated on it rather than adding a
  fifth copy.

Decisions recorded

• The autoload.files warm-up is declined, with numbers (0002 §3). It was never a
  correctness requirement — an unwarmed locator answers identically — so it only moves
  which message pays a one-off cost. At §8.1's measured ~4 MB/s that cost is ~35 ms here
  (~7 ms excluding PHPUnit's Assert/Functions.php, which is 111 KB of the 139 KB).
  Not worth ~105 lines, a second InitializedListener, and a non-obvious ordering rule.
  A reopen condition is named, and warm() changes no interface, so re-adding it later
  is free.
• ConstantName is already taken by Domain\ConstantName (class constants), which
  collides with §5.3's global-constant FQN type. Flagged to be decided before S3.8b
  rather than inside it.

Also corrects the name-type JIT note: NameKind already exists, and QualifiedName
lands in S3.7c rather than S3.8.

Docs only — no src/ or tests/ changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.55%. Comparing base (7c91158) to head (2c8c8f7).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #390   +/-   ##
=========================================
  Coverage     98.55%   98.55%           
  Complexity     1753     1753           
=========================================
  Files           115      115           
  Lines          4434     4434           
=========================================
  Hits           4370     4370           
  Misses           64       64           

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