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/0031-node-instance-naming.md

@@ -258,7 +258,7 @@ contributes one `ParamSpec` per param slot, named
 prefix by its own name and recurses. Slot order and arity are **unchanged** from
 today (the mirror test against the compiled flat-node order stays green); only
 the per-knob *string* changes (it gains the node segment and, where a
-`ParamAlias` used to relabel, reverts to `<node>.<factory-param>`). The compilat
+`ParamAlias` used to relabel, reverts to `<node>.<factory-param>`). The flat graph
 remains name-free and wired by raw index (C23): node names are construction-time
 identity and debug symbols, dropped at lowering, exactly as alias names were.