Model scoped and UnscopedRef lifetimes in the type system

[sailro]

Related to #3853. This is the type-system prerequisite requested during review of #3854.

tl;dr: ILSpy currently treats ScopedRefAttribute as a single boolean and has no semantic
model for UnscopedRefAttribute, implicit scopes, or the module's ref-safety rules. This PR
aligns the type system with Roslyn without changing decompiler output. A separate follow-up
will use this model to recover erased scoped local modifiers.

Problem

LifetimeAnnotation.ScopedRef only reports whether a parameter has
ScopedRefAttribute. That is sufficient to print the explicit scoped keyword, but not to
reason about effective lifetimes:

• ScopedRefAttribute represents ScopedRef on a by-ref parameter and ScopedValue on a
  by-value ref-like parameter.
• out is implicitly ScopedRef only for modules using
  [module: RefSafetyRules(11)].
• ref-like params collections are implicitly ScopedValue, while their compiler-emitted
  ScopedRefAttribute is redundant and must not be printed.
• UnscopedRefAttribute can remove an implicit scope.
• a struct receiver is implicitly ScopedRef; UnscopedRefAttribute may be placed on the
  method, property, or accessor.

Without these distinctions, a local-lifetime analysis cannot tell whether an argument may be
captured by a call.

Solution

• Add ScopedKind with None, ScopedRef, and ScopedValue.
• Extend LifetimeAnnotation with the explicit attribute-encoded scope and
  HasUnscopedRefAttribute, while preserving the current ScopedRef API.
  ScopedKind replaces the obsolete preview-era RefScoped and ValueScoped fields.
• Recognize System.Diagnostics.CodeAnalysis.UnscopedRefAttribute and
  System.Runtime.CompilerServices.RefSafetyRulesAttribute.
• Decode ScopedRefAttribute according to parameter ref-ness and ref-like type semantics.
  Conflicting or malformed annotations degrade to no explicit scope.
• Read exactly RefSafetyRules(11) from the module, matching Roslyn's old/new escape-rule
  selection.
• Add internal helpers for effective parameter scope and implicit struct-receiver scope,
  including method/property/accessor UnscopedRefAttribute.

The display path continues to use only the explicit scope. Implicit out, this, and
ref-like params scopes therefore do not introduce redundant scoped keywords, and
UnscopedRefAttribute remains visible as a normal attribute.

Tests

• explicit scoped ref and scoped ref-struct parameters
• implicit new-rules out
• [UnscopedRef] out
• legacy out from a module without RefSafetyRules(11)
• struct, class, static, method, property, and accessor receiver scopes
• ref-like params effective scope
• ScopedRef option disabled, including preservation of the raw attribute
• existing RefFields and ParamsCollections Pretty matrices remain byte-identical

Validation:

• Debug and Release solution builds: 0 warnings, 0 errors
• full Pretty suite: 1906 passed, 5 existing skips
• full solution test suite: 4887 passed, 15 existing skips

Review focus

The main review points are the public LifetimeAnnotation shape and the exact mapping of
Roslyn's declared versus effective scope rules.

• At least one test covering the code changed

[siegfriedpammer]

@dgrunwald this needs a design/review session