Reintroduce JetBrains [PublicAPI] as an IDE authoring hint on consumer-facing assemblies

[ChrisonSimtian]

Problem

PR #76 removed JetBrains.Annotations wholesale (~1450 [PublicAPI] + friends). Side effect: Rider/ReSharper now flag Fallout's public build-authoring surface as "unused" — because it is unused in-repo by design (external consumers call it, we don't). Contributors get persistent false-positive "unused symbol" noise on exactly the API we ship.

[PublicAPI] tells the IDE "this is intentional API, consumed externally — don't flag it." It's an IDE authoring hint only: zero runtime/semantic effect.

Not this issue: API-surface break detection / changelog gating. That's #410 / #530 (Roslyn PublicApiAnalyzers). Complementary and separate — one silences an IDE hint, the other guards the shipped surface.

Outcome

Consumer-facing ("outer") assemblies advertise their public surface as intentional API again, so:

• contributors stop getting false unused-symbol warnings on shipped API;
• consumers' own Rider/ReSharper sees our surface as API too (the annotation flows through the package, no PrivateAssets).

Acceptance criteria

• JetBrains.Annotations referenced so the annotation flows to consumers (no PrivateAssets).
• Outer/consumer-facing assemblies carry [assembly: PublicAPI] via a single allowlist (one source of truth), not ~1450 hand-placed attributes.
• Inner assemblies (Cli, Migrate, SourceGenerators, MSBuildTasks, Tooling.Generator, Persistence) get no blanket; genuine extensibility types there are annotated per-type ([PublicAPI] in source) — the hybrid split.
• An ADR records the decision (mechanism + outer/inner criterion) and goes up for public review.
• An encapsulation-audit doc tracks any public-by-accident types the pass surfaces.