Implement the first engine-backed Project Mode feature slice

Parent

Part of #19

What to build

Make the standalone stateful analysis engine useful for an editor on the initial Project Mode corpus: a host commits a Project Snapshot and document updates, requests diagnostics, hover/type information, definition or declaration navigation, and completion, and receives immutable revision-tagged results. Stale results must be observable and rejectable without depending on completion order.

Acceptance criteria

• A platform-agnostic managed API accepts immutable Project Snapshots, Active Target identity, per-document versions, and independent document updates.
• One stateful analysis session serves diagnostics, hover/type information, definition/declaration navigation, and completion for the agreed supported corpus.
• Results are immutable and carry Workspace Revision, Freshness, and Availability values.
• Concurrent edits demonstrate latest-input-wins publication; superseded results are never presented as current.
• The feature slice has differential tests against the accepted FCS corpus and explicit assertions for incomplete and unsupported constructs.
• The standalone library and its tests build without an editor, MSBuild, LSP transport, or FCS dependency.

Blocked by

• #23 — Define differential correctness for the supported F# corpus
• #24 — Prototype a NativeAOT-compatible incremental syntax and dependency core

Read the blocked prerequisites, issues #23 and #24, before starting this feature slice. Use their accepted corpus and incremental-core decisions to define the managed API and stateful session, then verify the acceptance criteria with differential and concurrency tests; done means the standalone library and tests build without editor, MSBuild, LSP, or FCS dependencies.