docs(audience-sdk): apply XML doc style to public surface (SDK-216)

[ImmutableJeffrey]

Summary

Adds XML doc comments to every public type and member of Immutable.Audience.Runtime, enables Immutable.Audience.Runtime.xml generation so Rider and DocFX can surface the docs, and turns on CS1591 enforcement so future undocumented public surface fails the build. Also strips em-dash characters from comments, log messages, and exception messages across the audience SDK runtime and tests so the source is consistent with the comment style.

What's documented

• ImmutableAudience static class plus 6 properties (Initialized, CurrentConsent, UserId, AnonymousId, SessionId, QueueSize) and 11 methods (Init, Track x2, Identify x2, Alias x2, Reset, DeleteData, SetConsent, FlushAsync, Shutdown).
• AudienceConfig class plus 11 public properties.
• AudienceError class plus AudienceErrorCode enum (5 cases).
• IEvent interface plus Progression, Resource, Purchase, MilestoneReached typed events with <inheritdoc/> on the interface implementations.
• ConsentLevel (3 cases), IdentityType (8 cases), ProgressionStatus (3 cases), ResourceFlow (2 cases) enums.
• DistributionPlatforms static class plus 5 string constants.

Style applied

• Property summaries are noun phrases. Method summaries are imperative.
• Sentence case, period at end. British spelling.
• <see cref> for sibling references; <seealso cref> for See Also; <param> per parameter; <returns> on tasks; <exception cref> on documented throws.
• <inheritdoc/> on IEvent implementations.
• <remarks> only where it carries consumer-facing nuance the signature does not (negative-space contracts, persistence guarantees, swallow contracts, handling semantics).
• Maintainer-only blocks (threading invariants, lock notes, IL2CPP rationale, internal phase commentary) stay as // line comments.
• Style ruleset lives at code-review/audience-comment-style.md.

Build infrastructure

• Audience.Runtime.csproj: adds <GenerateDocumentationFile>true</GenerateDocumentationFile> so the XML file ships beside the DLL on every build.
• CS1591 (missing XML comment on publicly visible type or member) is enabled with no NoWarn suppression.

Em-dash cleanup

• 57 em-dashes removed from runtime source (in // comments, Log.Warn messages, and ArgumentException messages).
• 55 em-dashes removed from test source (in // comments and assertion messages).
• Replacements use period plus new sentence, colon for "purpose: mechanism", parentheses for asides, semicolon for tight clause linkage. Per code-review/audience-comment-style.md §1.5.

No code behaviour changes. dotnet test passes (274 / 274; 1 pre-existing skip).

Linear: SDK-216