docs: document frontend/backend boundary rule and spec workflow

Add the "domain vocabulary lives in Rust" principle, the
check:boundary pre-commit gate, and a spec-writing section pointing at
the spec template and review checklist.

CLAUDE.md

@@ -39,6 +39,8 @@ only against the mirror if one exists; the canonical remote is
 
 - Frontend: `bun run check` and `bun run test` must pass.
 - Rust: `cd src-tauri && cargo fmt` then `cargo clippy`, plus `bun run test:rust`.
+- **Boundary**: `bun run check:boundary` must pass — no domain taxonomy (Jellyfin
+  item-type category sets) leaked into the frontend. See below.
 - **Traceability**: new requirement-implementing code must carry a `// TRACES:`
   comment (see below).
 - **Android source edits**: edit `src-tauri/android/src` (the canonical tree),
@@ -161,6 +163,25 @@ and [docs/build-release.md](docs/build-release.md).
 - **Graceful backend init.** If a native player backend fails to initialize, the
   app falls back to a no-op backend and emits `backend-init-failed` rather than
   crashing.
+- **Domain vocabulary lives in Rust.** The frontend is presentation-only and must
+  not encode Jellyfin's *taxonomy* — e.g. the set of item types that defines a
+  category like "Music". Send an opaque scope/enum across the boundary and let the
+  backend expand it. Single-type presentation (`itemType: "Movie"`, "this page
+  shows albums") is fine; a *category → set of types* mapping in `src/` is a leak.
+  `bun run check:boundary` is the tripwire; the real gate is the spec's layer
+  assignment. See [scoped-search-boundary.md](docs/specs/scoped-search-boundary.md)
+  for the incident this rule came from.
+
+## Writing specs
+
+New feature specs go in [docs/specs/](docs/specs/). **Start from
+[SPEC-TEMPLATE.md](docs/specs/SPEC-TEMPLATE.md)** — its "Layer assignment" section
+forces each piece of *logic* to be placed in the correct layer (Rust = domain,
+frontend = presentation) *with a reason*, which is what prevents boundary leaks.
+Before accepting a spec, run it past
+[SPEC-REVIEW-CHECKLIST.md](docs/specs/SPEC-REVIEW-CHECKLIST.md). Do **not** frame
+a spec around "no Rust changes required" — correct layer placement is the goal,
+not minimal backend churn.
 
 ## Conventions