Simplify MuxTableTarget, cheaper query hashes

[robknight]

Closes #527

Two related changes to the operation aux-table machinery, building on #504:

1. Cheaper query hashes. The custom-predicate and open-input-statement aux queries now hash over already-materialized component hashes instead of re-flattening wide statement structures. The custom query went from hashing a ~314-element flattening of (statement, op_type, op_args) to hashing statement_hash (4) + op_type (6) + op_arg_hashes (N*4), where those component hashes are reused from work the circuit already does (vec_ref_projected now returns the hash it computes; statement_hashes[i] is already available).

2. Collapse the aux-table indirection. With every entry now a 4-element query hash, the table no longer needs the generic hash-and-unhash layer it inherited. MuxTableTarget stores query hashes directly, get returns a bare HashOutTarget, and TableEntryTarget is removed. This addresses issue Simplify MuxTableTarget #527.

Collapsing the indirection drops the per-row OperationAuxTableTag and the per-verify tag check. That is sound because domain separation now lives entirely in the query hash: every hash_*_query helper routes through operation_aux_query_hash, which prepends a unique OperationAuxQueryKind.

Removing this indirection reduces gate count by 2200 or so.

[ed255]

LGTM! It's a very nice optimization to reuse the hashed statements and operation arguments for the queries to the aux table.

I've left 2 minor requests, please take a look.

[ed255]

This is a nitpick, but could you move this one to position 5 (and value 5), so that it matches the order in the table?
See

pod2/src/backends/plonky2/mainpod/operation.rs

Line 118 in 10e8cc9

Self::OpenInputStatement(i) => Self::table_offset_open_input_statement(params) + *i,

[ed255]

would you mind renaming this method to hash_open_input_statement_query? I realized this method uses the OperationAuxQueryKind::OpenInputStatement so it's more specific than just hashing a statement.