[M9] Stabilize Rust embedding API over compiler/session workflows

[Teakowa]

Parent: #10

Depends on: #55

Planning status: provisional until the M8 v1 gate is complete.

Goal

Expose Wright as a reusable Rust library for other projects without requiring subprocess CLI orchestration or access to internal IR layout.

Scope

• Define stable embedding entry points for project/source loading, compile, check, analyze, inspect/query, and target configuration.
• Reuse the M6 compiler/session driver rather than duplicating pipeline orchestration.
• Define owned request/config/result/diagnostic types appropriate for public use.
• Establish lifecycle/concurrency expectations for one-shot and reusable sessions.
• Keep internal HIR/WIR arenas and parser implementation details behind the public boundary.
• Define semver/stability expectations for embedding APIs after v1.

Non-goals

• Exposing raw mutable IR as the primary public API.
• Binding the embedding API to MCP, JSON-RPC, or another transport.
• Providing language-server protocol types.

Acceptance criteria

• An external Rust consumer can load supported inputs and perform core Wright workflows without invoking the CLI.
• Public APIs return structured owned results/diagnostics and do not require parsing terminal output.
• Internal compiler organization can evolve without unnecessarily breaking consumers.
• API stability/versioning policy is explicit and tested through an external-consumer fixture or equivalent.

[Teakowa]

Delivered in commit 03115d9 (wright-driver, docs/embedding.md).

• Stable embedding entry points: CompilerSession (idempotent load), SessionConfig/InputSpec/SourceKind/OutputFormat/Profile, typed Envelope<T> results — all reused from M6, not duplicated.
• Internal HIR/WIR arenas and parser internals stay behind the boundary; consumers import only wright-driver (the committed wright-consumer crate proves it).
• Lifecycle/concurrency documented (docs/embedding.md): one-shot and reusable sessions, idempotent loading.
• Semver/stability expectations recorded (additive within wright-result/v1/wright-embedding/v1; wright-embedding/v1 constant added).