Add microfluidic device and biology tutorials

[marpaia]

Add four MicroSimulator device/biology tutorials and a reproducible nutrient-validation experiment. Tutorials consume actual biomass growth divided by yield, use backward Euler for stiff transport, and restrict stationary resistance to fixed cells. The pillar mask uses the physical cylinder radius and the channel has its authored six-micrometer depth. The clock's secretion and enzyme field use intracellular amount; its parameters remain illustrative.

The controlled experiment couples 45 attached cells to nutrient and flow through a fixed physical kernel. Its implicit nutrient loss determines actual biomass gain, with explicit boundary-budget accounting. At spacing 1 it gains 31.00 biomass-volume units and has a 5.71 micrometer half-concentration depth. Halving spacing changes gain by 2.8%, halving timestep by 0.44%, and halving flow-refresh time by less than 0.001%; maximum balance error is below 0.008% of supply on CPU/Metal. Division, mechanics, and detachment are excluded from this controlled experiment and exercised separately by the tutorials.

Validation: revised tutorial build/run/checkpoint tests pass; the complete Python suite passes 327 tests (56 hardware/reference skips), and the separately configured pinned legacy checks pass 12 tests (5 CUDA skips). All 23 native tests and 10/10 analytic flow benchmarks pass on CPU/Metal. CUDA compilation passes without NVIDIA runtime validation. These are numerical checks, not biological calibration or performance claims.