Add guitars to tango: the Gardel-quartet nylon string

At Paul's direction. The real sampled nylon-string guitar (FluidR3) joins tango's acoustic combo: it takes the lead from the bandoneon on ~1/3 of seeds (hard staccato, same recipe — the guitar-quartet tradition) and joins the comping pool alongside the marcato strings.

• Sampler-pool additions only: one pick draw each, draw order unchanged, so only tango's own outcomes shift. Measured across 12 seeds: guitar appears in 6 (lead and/or comping). verify.sh PASS

▶ Play it: Ballroom Leviathan — a horn-and-piano genre that plays almost entirely sampled instruments https://stellate.app/?genre=bigband&seed=4224&path=814.14134,833.14167,795.14167&xdur=13.122

c67d53d https://github.com/aboard-io/stellate/commit/c67d53d0b4c04e1640637393f8e5f09990a986ed