docs,engine: drop coined "compilat", use FlatGraph / "flat graph"

"Compilat" (German "Kompilat") was a coined noun for the product of the
bootstrap compilation — neither English nor a natural fit. The runtime
artifact already has a code identifier for exactly this thing: the
`FlatGraph` struct (harness.rs). Replace the coinage with that identifier:

- prose mentions          -> "flat graph" (mirrors the type, reads plainly)
- definitional anchors    -> `FlatGraph` (C11, C23, the running-graph line)
- `render_compilat`       -> `render_flat_graph` (historical render symbol;
                             keeps the `render_blueprint` / `render_flat_graph`
                             source-vs-product pairing)
- `intra-compilat`        -> `intra-graph`
- `from_compilat` (test)  -> `from_flat`

"compilation" / "re-compilation" / "bootstrap-as-compilation" (the process,
ordinary English) are deliberately left untouched. Behaviour-preserving:
only comments, design ledger, specs/plans, one test-local variable and its
assert messages change. Full workspace test suite green; clippy clean.

crates/aura-engine/src/builder.rs

@@ -2,7 +2,7 @@
 //!
 //! A fluent, additive authoring surface that wires a blueprint by typed node
 //! handles and port/field *names*, resolving them to the raw-index `Composite`
-//! at a single fallible `build()`. The compilat stays index-wired (C23): names
+//! at a single fallible `build()`. The flat graph stays index-wired (C23): names
 //! are resolved at the authoring boundary and never reach `FlatGraph` — the same
 //! posture param-name resolution already has (`Binder`, blueprint.rs). Resolution
 //! mirrors `PrimitiveBuilder::bind`'s collect-then-reject (node.rs), but returns