Quality: Phase in XML documentation enforcement (CS1591)

[mikemcdougall]

Problem

XML documentation warnings are globally suppressed, so public APIs can ship without docs.

• Suppression in Directory.Build.props:12 (<NoWarn>...CS1591</NoWarn>).

Goal

Phase in documentation enforcement without blocking short‑term delivery.

Plan

1. Phase 1: Turn CS1591 into warning for Honua.Core (or selected public assemblies) and document new public APIs.
2. Phase 2: Expand to additional projects (Server/Postgres) once baseline docs are improved.
3. Phase 3: Promote to error for public APIs.

Tasks

• Remove global CS1591 suppression.
• Add project‑level settings to scope enforcement per assembly.
• Document existing public APIs in Honua.Core as a baseline.
• Update contributing docs to require XML docs for new public types/members.

Acceptance criteria

• CS1591 enforced for Honua.Core at minimum.
• New public APIs require XML docs.
• Build remains green with documented public surfaces.