Migrate NamespaceCandidates onto the SymbolSource seam

[Firehed]

Slice S2.3 — Migrate NamespaceCandidates to childrenOf

Slice: S2.3 (docs/architecture/build-manifest.md, Wave 1)
Plan step: docs/architecture/0002-execution-plan.md Step 2 (§5.5
consumer-migration table; §5.3 name-type model) — a consumer migration onto
the S2.1 seam, no behavior change.
Depends on: S2.1 (SymbolSource / SymbolSink + delegating facade, #374)
— merged.
RFC sections: 0001 §4.2 (Symbol Discovery Authority), §4.4 (FQN-based
knowledge queries), §5.1 (typed identifiers).

Routes NamespaceCandidates' namespace enumeration through
SymbolSource::childrenOf instead of naming NamespaceCatalog directly. Each
of the four call sites now wraps the namespace string in a NamespaceName
(the typed identifier the manifest flagged as needed by this slice, already
landed in S2.1). The consumer is constructed with the one shared
DelegatingSymbolSource facade wired in Server.php in S2.2.

Behavior preservation

The plan requires this migration be identical. The facade's childrenOf
unwraps NamespaceName and delegates to the same NamespaceCatalog the
consumer used before, so no enumeration result changes. The frozen
ChildrenOfParityTest golden (children-of.json) is unchanged — this slice
does not touch the enumeration surface itself — and the full
NamespaceCandidates unit suite plus the CompletionHandler namespace-
navigation cases stay green across every path (absolute, use, bare descend,
inline/node, multibyte range).

Acceptance criteria

• NamespaceCandidates reads namespace enumeration through
  SymbolSource::childrenOf, no longer naming NamespaceCatalog directly.
• The string namespace is carried as a NamespaceName typed identifier at
  each call site (RFC 1 §5.1).
• Offered candidates are identical (existing NamespaceCandidatesTest and
  CompletionHandlerTest coverage green).
• The ChildrenOfParityTest golden is unchanged (the enumeration surface
  is untouched).
• The shared facade is reused; construction routes through the S2.2
  DelegatingSymbolSource instance in Server.php.
• composer test green (PHPStan + tests + PHPCS).

Not in this slice

• The §4.2 enforcement rule forbidding direct
  SymbolIndex / ClassRepository / NamespaceCatalog names outside a
  backend lands in S2.6 (scoped-exempt), once the remaining consumers
  (S2.4, S2.5) have migrated.

Candidate closes (pending review verification)

None — the manifest lists no Closes for S2.3.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.14%. Comparing base (e2cf6b0) to head (86e3fa7).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #376   +/-   ##
=========================================
  Coverage     98.14%   98.14%           
  Complexity     1711     1711           
=========================================
  Files           107      107           
  Lines          4308     4308           
=========================================
  Hits           4228     4228           
  Misses           80       80           

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