Target-aware attribute completion filtering

[Firehed]

Summary

Filter attribute-name completion by the attribute's declared targets, so a #[
position only offers attributes valid there — e.g. don't offer a
TARGET_METHOD-only attribute on a class, or a TARGET_CLASS-only attribute in a
function-parameter position. This also covers suppressing completion entirely where
#[ is not a syntactically valid attribute position at all (see below).

Background

The initial attribute completion work added attribute-name completion (#[Ro →
attribute classes only) via ClassCandidateFilter::Attribute +
CodeResolver::isAttribute. That filter is target-agnostic: every confirmed
attribute is offered at any #[ on the line, regardless of position. This is the
"less sophisticated version first" explicitly called out in the parent issue.

Because detection is text-only, it also offers attributes after a #[ that appears in
a syntactically invalid spot (e.g. an expression position like $x = #[Fo). Deciding
"is this even a valid attribute position, and if so what target?" is the same
position analysis — so both concerns are resolved here rather than by bolting a second
validity check onto the text classifier.

Work

• Capture the attribute's target flags (the #[Attribute(Attribute::TARGET_*)]
  bitmask; #[Attribute] with no argument defaults to TARGET_ALL) on
  ClassInfo, populated from both the AST factory (attrGroups → the Attribute
  attribute's argument) and reflection.
• Detect the current attribute target at the cursor (class / method / property /
  parameter / constant / function / enum-case), and detect the absence of a valid
  attribute target. This needs position context beyond the text classifier — an
  AST-first path with a text fallback, consistent with the member/call detection seam.
  The text classifier deliberately stays resilient (see docs/features/completion.md),
  so target detection should layer on rather than convert the classifier to AST.
• Narrow the attribute filter (or a new CodeResolver predicate) by the detected
  target, and offer nothing when no attribute target applies.

Acceptance

• A TARGET_METHOD attribute is offered on a method but not on a class.
• TARGET_ALL / unflagged attributes are offered everywhere an attribute is valid.
• No attributes are offered where #[ is not a valid attribute position (e.g. an
  expression/statement position inside a body).
• When the position is valid but the target cannot be determined, fall back to offering
  all attributes (optimistic) rather than hiding valid completions.

Part of #252 (parent #311).

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Issue body written by AI, from a design discussion with the maintainer.