Add support for non-uniform focal point hit counts

Currently, pulses in a sonication sequence are distributed equally across all focal points in a round robin pattern. In multifocus treatments, focal points can receive different effective acoustic energy per pulse, so equal pulse distribution can leave some foci undertreated relative to ther others. Allowing more pulses to be allocated to weaker foci could raise the min ISPTA across focal points and improve treatment uniformity.

Main proposed changes:

• Solution.focal_hit_counts: add a new field storing the number of pulses directed to each focal point
• Protocol.calc_solution: add new focal_hit_counts and optimize parameters. focal_hit_counts accepts an explicit allocation. optimize=True computes an optimized allocation automatically
• optimize_hit_counts(): add a helper that maximizes the min ISPTA across focal points, subject to TIC limits from param_constraints
• SolutionAnalysis.per_focus_tic: add a new field exposing per-focus TIC values
• weighted intensity aggregation in calc_solution and Solution.get_ispta

Please let me know if this sounds reasonable and if you'd be open to a PR. I'd love to contribute. Thanks!

Start by reading Solution, Protocol.calc_solution, SolutionAnalysis.per_focus_tic, and the existing param_constraints handling. Trace how focal points are currently assigned pulses and how ISPTA and TIC are aggregated. Done should include explicit and optimized focal-hit allocations, constraint-aware optimization, per-focus TIC reporting, and weighted intensity calculations.