docs+scaffold: engine/project split (C16-C18), aura-std, usage doc

Refine the structure per interview: aura is the reusable engine, research projects are separate external repos depending on it (C16). Add aura-std (universal standard-node tier) to the workspace; remove nodes/ from the engine (project concept). Record authoring-surface = Claude Code + skills pipeline, no embedded coding-LLM (C17), and project management = one-repo-one-project + Aura-native run registry (C18). Add CLAUDE.md invariants 9-10, code_roots -> [crates], and docs/project-layout.md documenting the day-in-the-life and project repo layout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/aura-std/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "aura-std"
+edition.workspace = true
+version.workspace = true
+license.workspace = true
+publish.workspace = true
+
+[dependencies]
+aura-core = { path = "../aura-core" }

crates/aura-std/src/lib.rs

@@ -0,0 +1,17 @@
+//! `aura-std` — the standard node library shipped with the engine.
+//!
+//! The universal, batteries-included building blocks every project gets for
+//! free: common indicators (SMA, ATR, RSI, …), resamplers, the `SessionNode`,
+//! standard combinators, and broker profiles. Depends only on `aura-core` (the
+//! shared contract).
+//!
+//! This is the top tier of the three-tier node-reuse model (contract C16):
+//!
+//! - `aura-std` — universal blocks, here, ship with the engine;
+//! - shared node crates — cross-project-reusable blocks, in their own repos,
+//!   pulled in as cargo git dependencies;
+//! - project-local `nodes/` — experimental, project-specific blocks.
+//!
+//! Types are intentionally absent until the first spec needs them (the
+//! walking-skeleton milestone). This crate documents intent; it does not yet
+//! define API.