docs(ledger): record the analysis meta-level as an open thread pointing to #109

Add an "Open architectural threads" entry that ties together aura's differentiator
(C21): the composable-orchestration rebuild (sweep/MC/walk-forward as composable
tools, not hard-wired CLI verbs) + the project-as-crate authoring layer (aura new /
experiment-builder API / Aura.toml / cdylib loading, C16/C17). Records the
load-bearing open fork — "driven via the CLI" = running a Rust-authored experiment
(compatible with C16/C17/C22) vs wiring nodes through the CLI itself (a deliberate
amendment to C17/C22/C9) — to be resolved in the brainstorm before any cycle.

Keeps the meta-level in focus in the always-loaded design memory; #109 is its
tracker home. No design decided here (the fork is recorded, not resolved).

refs #109

docs/design/INDEX.md

@@ -1176,6 +1176,21 @@ load-bearing in the flat graph.
   experiment-builder API surface (harness wiring, structural axes, sweep
   combinators) and `Aura.toml`'s schema (data paths, instrument/pip metadata,
   default broker & window, runs dir) are not yet designed.
+- **The analysis meta-level — composable orchestration + project-as-crate authoring
+  (tracked: #109).** aura's differentiator (C21) has two unbuilt halves: (1) the
+  orchestration axes (sweep / Monte-Carlo / walk-forward) becoming *composable tools*
+  a user wires into an analysis workflow, rather than today's hard-wired CLI verbs
+  (`sweep_family` / `walkforward_family` in `aura-cli`); and (2) the project-as-crate
+  authoring layer above (the `aura new` / experiment-builder API / `Aura.toml` thread
+  + cdylib loading, C16/C17). On the meta-level, authoring a strategy is *wiring
+  existing nodes* (C9 composition) + *defining the analysis framework* (C21), not
+  writing new nodes; the user wants it driven via the CLI, later the interactive
+  server / playground (C14/C22). **Load-bearing open fork — resolve in the brainstorm
+  before any cycle:** "driven via the CLI" = (a) *running* a Rust-authored experiment
+  via the CLI / playground (compatible with C16/C17/C22) vs (b) *wiring nodes through
+  the CLI itself* (a deliberate amendment to C17's no-DSL, C22's no-UI-topology, and
+  C9's no-by-name-node-registry). Deferred by the user (2026-06-22); #109 is its
+  durable home until promoted to a milestone seeded by that brainstorm.
 - **`aura-std` contents** — the crate exists (doc-only); which universal blocks
   land first follows the walking-skeleton's needs.
 - **`strategies/` split** — a later split, *inside a project*, of top-level