refactor: model static and $this as bounded types in the type system

[calebdw]

Add PhpType::StaticType(Atom) and PhpType::ThisType(Atom) variants that preserve late-static-binding semantics instead of flattening static/$this to a bare class name. StaticType carries the bound class ("at least this class or a subclass"), ThisType is more specific ("the exact runtime instance type").

The subtype chain is ThisType(A) <: StaticType(A) <: Named(A).
Display: static(Foo), $this(Foo) -- shows the bound class.

Production sites updated:

• replace_self(fqn) now delegates to resolve_self_refs_bounded() so static -> StaticType(fqn) and $this -> ThisType(fqn); replace_self_with_type(&receiver) is unchanged (preserves full generic receiver types for accurate chain resolution)
• Subject resolution: $this -> ThisType, static -> StaticType
• First-class callable partial application: preserves static/this
• Template substitution: preserve_static path uses bounded types
• new static() -> StaticType instead of Named
• Diagnostic param checking: resolve_self_refs_bounded() produces bounded types so class-string is properly validated

Subtype checking updated:

• StaticType(A) <: Named(A) and ThisType(A) <: Named(A)
• ThisType(A) <: StaticType(A)
• base_name(), top_level_class_names(), collect_class_names() all handle the new variants

Peripheral sites updated:

• return_type_is_mixin_self handles StaticType/ThisType
• is_simple_php_type handles StaticType/ThisType
• Union member deduplication handles StaticType/ThisType

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 63.81910% with 72 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/php_type/transform.rs	53.19%	66 Missing ⚠️
src/diagnostics/type_errors/mod.rs	76.92%	3 Missing ⚠️
src/php_type/mod.rs	85.71%	1 Missing ⚠️
src/php_type/subtype.rs	88.88%	1 Missing ⚠️
src/type_engine/call_resolution/template_subs.rs	75.00%	1 Missing ⚠️

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