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/plans/0015-node-param-declaration.md

@@ -14,7 +14,7 @@ params into one flat, path-qualified, inspectable param-space.
 declare their tunable knobs. `aura-engine` gains a read-only `Blueprint::param_space()`
 that walks the graph-as-data (using the already-public `Composite::name()` as path
 prefix) in the same deterministic depth-first order `lower_items` uses — a parallel
-projection that leaves `compile`/`inline_composite` (and the compilat) untouched.
+projection that leaves `compile`/`inline_composite` (and the flat graph) untouched.
 
 **Tech Stack:** Rust workspace — `aura-core` (node contract), `aura-std` (7 nodes),
 `aura-engine` (blueprint/compile). Param identity is positional (slot), name a
@@ -320,7 +320,7 @@ the build names any remaining `NodeSchema` literal missing `params`, add
 Run: `cargo test -p aura-engine`
 Expected: PASS — all pre-existing tests (incl. the bit-identical
 `composite_sma_cross_runs_bit_identical_to_hand_wired` and the golden render tests)
-stay green: the compilat is unchanged, only schema literals gained an empty field.
+stay green: the flat graph is unchanged, only schema literals gained an empty field.
 
 ---
 
@@ -515,7 +515,7 @@ and `LinComb::schema` are clippy-clean).
 - **Do not touch** `compile`, `inline_composite`, `lower_items`, or any edge/wiring
   logic. `param_space()` is a *parallel* read-only projection that mirrors the
   inliner's traversal order; it must not share or alter it. The spec's correctness
-  rests on the compilat staying bit-identical (every existing golden / bit-identical
+  rests on the flat graph staying bit-identical (every existing golden / bit-identical
   test stays green by construction).
 - **fieldtests/ are out of scope** — they are excluded crates (own `Cargo.toml`),
   not in `--workspace`, and are frozen cycle-archive snapshots. They construct the