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.

CLAUDE.md

@@ -114,10 +114,10 @@ design decision, not a refactor, and belongs in the ledger.
     blueprint). The harness — sources + strategy + broker node(s) + sinks — is
     the root sim graph, itself bootstrapped, and is C1's disjoint unit; its
     structural axes form the experiment matrix, its tuning params the sweep.
-    **The bootstrap is a compilation** to a flat, type-erased *compilat*, wired
+    **The bootstrap is a compilation** to a flat, type-erased `FlatGraph`, wired
     by raw index (composites inline — a composite is an authoring-level node, not
     a runtime object; names survive only as non-load-bearing debug symbols, as
-    today). The compilat is the target of **behaviour-preserving** optimisation,
+    today). The flat graph is the target of **behaviour-preserving** optimisation,
     C1 the correctness invariant — intra-graph (CSE/DCE) and across a sweep family
     (sweep-invariant sub-graph computed once, shared via C11/C12). See C9/C19/C23.
 12. **The World is the product; the playground is a trace explorer.** Three