docs: capture the empty [workspace] table gotcha for nested project crates

A project crate that path-depends on the engine and lives under the engine
repo (common during authoring) is rejected by cargo unless its Cargo.toml
carries an empty [workspace] table. Surfaced repeatedly by the cycle-0006 and
cycle-0007 fieldtests as a non-obvious onboarding one-liner.

Document it on the consumer-facing surface (project-layout, the "A project
repo" section) so a nested-project author can predict and fix it. The aura new
scaffolder will emit the line once it exists (folds into the CLI work); the
docs capture closes the actionable half now.

closes #9

docs/project-layout.md

@@ -32,7 +32,8 @@ A project is always a Rust program built on the engine.
 ger40-lab/                 # your research project — a separate Rust crate (cdylib)
 ├── Aura.toml              # STATIC context only: data paths, instrument/pip
 │                          # metadata, default broker & window, runs dir (no logic)
-├── Cargo.toml             # cdylib; depends on aura-core/aura-std (+ shared node crates)
+├── Cargo.toml             # cdylib; depends on aura-core/aura-std (+ shared node crates);
+│                          # carries an empty [workspace] table (see note below)
 ├── CLAUDE.md              # the project's own skills wiring (authoring discipline)
 ├── .claude/dev-cycle-profile.yml
 ├── nodes/                 # project-local node & strategy blueprints (Rust)
@@ -42,6 +43,22 @@ ger40-lab/                 # your research project — a separate Rust crate (cd
 └── (frozen bots, …)
 ```
 
+> **The empty `[workspace]` table.** A project crate is its own cargo workspace,
+> never a member of the engine's. This matters the moment a project lives *under*
+> the engine repo (common while authoring, before it moves to its own repo):
+> cargo otherwise walks up, finds the engine's root manifest, and refuses to
+> build —
+>
+> ```
+> error: current package believes it's in a workspace when it's not:
+> ... to keep it out of the workspace, add an empty [workspace] table ...
+> ```
+>
+> The fix is cargo's own hint: an empty `[workspace]` table in the project's
+> `Cargo.toml`. It pins the crate as a standalone workspace root and costs
+> nothing when the project later sits in its own repo. The future `aura new`
+> scaffolder will emit this line; until then, add it by hand.
+
 ## Where reusable nodes live (three tiers)
 
 Everything that plugs into the engine is fractally a `Node`. Reuse is plain