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

[zzylol]

Summary

Records the placement decision settled in #222: common sub-expression
elimination (CSE) detection is a primary pass over the pre-ASAP
QueryExpr IR (asap-types::pre_asap), not something asap-aware-mapping
does. The algorithm itself is tracked separately in #223 (design doc,
implemented by a different workstream); this PR only corrects the
documentation to reflect the decision.

asap-aware-mapping's own module doc (crates/asap-aware-mapping/src/lib.rs)
previously claimed "which shared sub-expressions to hoist" as this crate's
job — that was aspirational text with no backing implementation, and per
#222 it's also simply the wrong layer for CSE detection to live. This PR
corrects that doc comment to say:

• Primary CSE detection lives in asap-types::pre_asap (design in Design: pre-ASAP structural CSE via bottom-up hash-consing over Rc<QueryExpr> #223),
  running before a tree ever reaches bind::implement_tree.
• asap-aware-mapping may eventually run a secondary, narrower CSE pass
  of its own over an already-implement_tree'd SummaryNode DAG —
  recognizing post-ASAP-only sharing such as Quantile(x, 0.99) and
  Quantile(x, 0.95) sharing one built sketch, read out twice — but that
  is downstream of, and not a substitute for, the primary pre-ASAP pass.

crates/asap-aware-mapping/src/cost_model.rs's module doc ("workload-level
CSE credit" as a planned cost-model input) was checked and left unchanged —
it only speaks to cost-model ranking, which is downstream of detection, and
doesn't claim this crate does detection itself. docs/*.md were checked
too; none currently discuss CSE placement, so there was nothing to correct
there.

I've also posted a comment on #212 (the parent "add CSE" issue) recording
this decision against its own "Where it runs." open question.

Scope

No runtime/behavior changes — module-doc comment only. cargo build --workspace --all-targets, cargo test --workspace, cargo fmt --all -- --check, and cargo clippy --workspace --all-targets --all-features -- -D warnings all pass unchanged.

Closes #222

Test plan

• cargo build --workspace --all-targets
• cargo test --workspace
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings

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