docs(asap-aware-mapping): correct module doc on where CSE detection runs

crates/asap-aware-mapping/src/lib.rs

@@ -3,8 +3,22 @@
 //! This crate sits between the language-agnostic IR ([`asap_ir`]) and
 //! any runtime: it consumes pre-ASAP [`QueryExpr`](asap_types::pre_asap::QueryExpr)
 //! trees and makes the cost-aware decisions the pre-ASAP IR deliberately
-//! leaves open — which shared sub-expressions to hoist and which sketch (if
-//! any) realises each approximate intent.
+//! leaves open — which sketch (if any) realises each approximate intent.
+//!
+//! **Common sub-expression elimination (CSE) is not this crate's job.**
+//! Detection is a primary pass over the pre-ASAP `QueryExpr` IR itself
+//! (`asap_types::pre_asap`, design tracked in issue #223), run before a
+//! tree ever reaches [`bind::implement_tree`] — see issue #222 for why
+//! (batch query optimization needs to see shared work across a
+//! `QueryWorkload` before summary binding, not after). This crate may
+//! eventually run a second, narrower CSE pass of its own over an
+//! already-[`implement_tree`](bind::implement_tree)'d
+//! `SummaryExpr`/`SummaryNode` DAG, recognizing sharing that's invisible
+//! at the pre-ASAP level by construction — e.g. `Quantile(x, 0.99)` and
+//! `Quantile(x, 0.95)` are structurally distinct `AggIntent`s but can
+//! still share one built sketch, read out twice. That post-ASAP pass is
+//! secondary to, and downstream of, the primary pre-ASAP pass, not a
+//! replacement for it.
 //!
 //! It depends only on the IR crate, never on a front end — the layering
 //! invariant (arrows point up) holds here too.