Update benchmark flow to use parametric n and generated indices

[pesap]

Problem

Our benchmark flow should support parametric n and generated index domains instead of fixed one-off fixtures.

Right now dense-lp is fixed around explicit indices. Once we have:

• parametric analysis inputs, and
• generated set construction,

we should update benchmark flow to use them.

Goal

Update benchmark/examples so dense benchmark can be expressed with a variable n and generated indices, instead of fixed files for each size.

Example target shape:

• benchmark runner passes n
• Arco model/scenario builds i and j from that size
• benchmark sweeps n = 25, 100, 500, ...

Blocked by

• Support parametric analysis inputs for scenario-sized benchmark models #163
• Support generated set construction for compact range domains #164

Acceptance

• benchmark flow can vary n without external KDL generation
• generated indices replace fixed dense benchmark index fixtures
• benchmark runner can sweep multiple n values cleanly