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.

docs/specs/0015-node-param-declaration.md

@@ -44,10 +44,10 @@ Two layers, both resting on machinery that already exists:
    the deterministic depth-first item order that `lower_items` (the inliner)
    already uses. **`compile`/`inline_composite` are not touched**: the param-space
    is a separate read-only projection of the blueprint (a further C9
-   introspection, like `nodes()`), not a change to the flat compilat.
+   introspection, like `nodes()`), not a change to the flat graph.
 
 The **identity** of a param is **positional** — its slot index in the flat
-param-space — exactly as the compilat wires by raw index, not by name (C23). The
+param-space — exactly as the flat graph wires by raw index, not by name (C23). The
 name is a **non-load-bearing debug symbol**, path-qualified at aggregation time
 (`strategy.combine.weights[0]`) so a reader can trace a knob back to its node.
 
@@ -270,7 +270,7 @@ whose kind could mismatch (that check lands with the bind in #31).
   param-space; an empty blueprint yields an empty space.
 
 Build/test/clippy green across the workspace; existing compile/bootstrap/run and
-render tests stay green (the compilat is unchanged — `compile`/`inline_composite`
+render tests stay green (the flat graph is unchanged — `compile`/`inline_composite`
 are not touched, so every existing bit-identical and golden-snapshot test is
 unaffected by construction).
 
@@ -292,7 +292,7 @@ applies, read against aura's domain invariants:
    Determinism (C1) holds — aggregation order is a pure structural function.
    Topology-invariance (C19) holds — a vector knob's arity `N` is topology-fixed
    (it equals the node's input arity), declared as `N` fixed entries, never a
-   sweep param. The compilat is untouched (C23) — params are a read-only
+   sweep param. The flat graph is untouched (C23) — params are a read-only
    projection, not a change to the wired graph; every existing bit-identical test
    stays green by construction.
 
@@ -300,7 +300,7 @@ applies, read against aura's domain invariants:
 
 - **Param-set binding / injection (#31)** — binding an injected typed value-vector
   to the param-space slot-by-slot, with the kind-check that declaration defers.
-- **Sweep enumeration (#32)** — building a family of compilats over the
+- **Sweep enumeration (#32)** — building a family of flat graphs over the
   param-space.
 - **The run-supplied search-range (#32 / C20)** — which subset/grid a run sweeps
   lives in the experiment-builder, not in the node (the param declaration carries