[Refactor] Unify indirect-access offset paths, add example tests

[yuhaohaoyu]

Scope/Background

PR #188 (and its predecessors #147, #183, #193) delivered a stable blocked-indirect fast path for emulating kernels aligning with MoE, paged-attention, and multi-index gather/scatter patterns. While the offsets= refactor unified read/write steps, two independent offset-computation paths remain: the general (item-wise) path (_resolve_idx_reads → _build_indirect_coords → _flat_memory_offsets) and the fast path (_prepare_dep_var_sub_space → _runtime_read_and_expand_sub_space → _gen_offsets_vso_space_via_broadcast), where both produce the same artifact — a flat int64 offset array — via completely separate code.

Problem to Address (according to Lars' final comments fro PR 188)

• **M1 (dual paths):**The dual-path existence hurts maintainability. The shared-view (shared subscript array) fix in _build_indirect_coords (PR Blocked-indirection load store, fast path #188) is a concrete example for reference when addressing the issue.
• Test gap: No existing MLIR example exercises the fast path end-to-end. All indirect-access fixtures (including paged-attention/paged-tensor) fail the has_direct_expr if-gate, making the fast path unreachable from example tests. The fix is needed for the execution of notebooks, etc.

Scope/Proposal

• Refactor both pipelines (element-wise and blocked-index) to share a common offset-computation interface, where the fast path is a vectorized specialization of the general path rather than a parallel implementation. Lars's recommendations file outlines the structural approach.
• Add 2–3 MLIR integration fixtures (MoE-style, paged-attn-style) that satisfy all fast-path gates, confirming the blocked-indirect path is exercised by pytest without synthetic Python-only test scaffolding.

Acceptance Criteria

• Single offset-computation interface (internal) for the element-wise and blocked-index emulation paths for indirect-accesses.
• Example tests that emulate MoE and PagedAttn kernels with the blocked-index emulation path lit.

Out of Scope