Single-source the MEMW_R fill and the BITWISE type↔column map

[MauroToscano]

Second follow-up from the #786 review, stacked on #790. Removes every "keep the two in sync"-by-comment pair the optimization introduced, with no behavior change (bucket routing, trace cells, and multiplicities are unchanged).

MEMW_R

• RegRow and the direct column fill move into memw_register.rs; generate_memw_register_trace becomes a thin wrapper (ops → RegRow::from_memw → shared fill). The unit-tested generator and the production fast path now run the same code, so they cannot drift — previously production ran an untested duplicate while the tests exercised the legacy copy.
• RegRow::new is the single definition of the row encoding (halved address, masked old_ts_lo); the walk fast path and from_memw both delegate to it.
• The IS_HALFWORD collector moves next to bus_interactions(), so the lookup each row sends and the lookup counted live in the same module.

Routing

• New MemwRoute + classify_memw shared by MemwBuckets::push, the push_reg_access fallback (now delegating to push), and count_table_lengths' partition_memw — the sizing pass and trace generation can no longer classify differently.
• Drops the single-use push_register/push_register_row helpers.

BITWISE

• BitwiseOperationType::ALL is the single origin of NUM_LOOKUP_TYPES, and type_mu_column(i) = mu_column(ALL[i]) — the arithmetic MU_MSB8 + type_idx shortcut and its column-contiguity assumption are gone rather than guarded.
• The runtime histogram_column_map_guard test is replaced by a compile-time bijection assert: reordering lookup_type_index or forgetting to extend ALL for a new variant is now a compile error instead of a test failure someone has to remember to run.
• bump()'s indexing comment corrected: in release an out-of-domain z would silently mis-count in another type's lane rather than panic (the old per-op path panicked); what actually upholds the invariant is that every BitwiseOperation constructor masks or debug-asserts z < 16.

Testing

• Full cargo test -p lambda-vm-prover --lib: 503 passed, including test_prove_verify_with_memw_register and test_prove_elfs_test_bitwise_8 end-to-end prove+verify.
• The 5 remaining failures (test_pure_commit_rust etc.) are missing rust-guest ELF fixtures in my environment (make compile-programs-rust) — they fail at file-read before any prover code runs.
• cargo clippy and cargo fmt --check clean.