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/0031-node-instance-naming.md

@@ -485,7 +485,7 @@ In the C23-related prose at :359-366 and :813-851: strike the `ParamAlias`
 param-overlay from the named-projection set (the param projection is retired;
 `Role.name` input roles and `OutField.name` outputs remain). Note that node names
 join the same non-load-bearing debug-symbol class (construction-time identity,
-dropped at lowering; the compilat stays wired by raw index). Remove the sentence
+dropped at lowering; the flat graph stays wired by raw index). Remove the sentence
 referencing the dangling-alias `BadInteriorIndex` compile check (:362-364) — that
 path is gone with the overlay.