[milindsrivastava1997]

Closes #1191. Follows #1096.

Why

Pass 1 no longer imports Pass 2 (#1192), but Pass 2 still imported Pass 1's error type and two of its helpers, so the passes were not independent modules.

What / how

• Errors: Pass 2's rewrite helpers (asap_aware_cse) return a new crate-private CseError with the three variants they use. No code matches its variants: callers stringify it into Pass2RuleError, drop the candidate, or call .ok().
  • PerComputationCandidateError loses TooManyCandidates, which only Pass 2 produced.
  • ErrorBuildingLogicalASAPDagsFromChosenCandidates::Recompose wraps SchemaDerivationError, the only error it ever held.
• Shared helpers: what both passes need are IR operations, so they move to asap-types as Field::column_ref, FieldDataType::exact and Reduction::partitions_over. Field::column_ref also replaces two copies of the same logic: inline in schema/topk.rs, and Pass 1's schema_column_ref.
• rolled_up: it builds its Sum/Min/Max exact accumulator inline (reads the value column, no item, UnknownOrSigned) and keeps Pass 1's check that the column resolves. Pass 1's exact_accumulator builds the same node for this case, but it would have pulled ~300 lines and Pass 1's error type into a shared module.
• tests/pass_layering.rs: walks each pass's directory recursively, matches pass1:: / pass2:: in production lines, and now tests both directions.

Error enums per pass

Pass	Enum	Visibility	Variants
1	PerComputationCandidateError	public	Structure, AssignedPhysicalAnnotation, ContainsAsap, InvalidAccuracy, InvalidChoice, Unsupported (this PR removes TooManyCandidates)
1	CandidatePopulationError	public	RootDependenciesChanged, InvalidTarget, InvalidAbsorption
1	PerComputationLogicalMemoCandidatesError	public	PerComputation, Population, Sizing (wraps the two above and SchemaDerivationError)
2	Pass2RuleError	public	MismatchedGroups, InvalidGroupReference, TooManyGroups, CyclicDependencies, AlreadyExpanded, ScanLabelWidening, StateConfiguration, ReuseRule
2	ErrorBuildingLogicalASAPDagsFromChosenCandidates	public	NoSuchCandidate, ChildNotInCandidate, Cycle, Recompose (this PR changes its payload to SchemaDerivationError)
2	CseError (new)	crate-private	Structure, Unsupported, TooManyCandidates

The rules turn a CseError into text inside Pass2RuleError (ReuseRule, StateConfiguration, ScanLabelWidening), or drop the candidate.

Before this PR

$ cargo test -p asap-logical-asap-optimization --test pass_layering
Pass 2 imports Pass 1:
src/pass2/memo/enumeration.rs:25: use crate::pass1::per_computation_candidates::PerComputationCandidateError;
src/pass2/asap_aware_cse.rs:124: use crate::pass1::per_computation_candidates::PerComputationCandidateError;
src/pass2/asap_aware_cse.rs:1839: use crate::pass1::per_computation_candidates::{exact_accumulator, partitions_over};

After this PR

Both pass1_does_not_import_pass2 and pass2_does_not_import_pass1 pass. Test modules may still use the other pass to build fixtures.

Key code interfaces added / modified

• Public: PerComputationCandidateError::TooManyCandidates removed. ErrorBuildingLogicalASAPDagsFromChosenCandidates::Recompose now holds SchemaDerivationError. No caller in the workspace matched either.
• Public, asap-types: Field::column_ref, FieldDataType::exact and Reduction::partitions_over are new.
• Crate-private: CseError is new. Pass 1's schema_column_ref is removed.

Major algorithm changes

None.

Snapshot changes

None.

Testing

• New unit test partitions_over_matches_keys_by_column in asap-types.
• rollup_from_a_finer_exact_sum and extrema_and_integer_counts_roll_up cover the inline accumulator. Making it return None fails 5 rollup tests.
• The tests of logical-asap-optimization, devtools (corpus snapshots included), planner, and the Stage 1 integration tests pass, as do asap-types'. Clippy is clean.

Out of scope

Splitting the passes into separate crates (#1209).

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[zzylol]

In the PR body, could you also list out the error enum per pass? so we can check the coverage. Thanks!