[FXC-5592] Results model for (TimeAveraging)ForceDistributionOutput

[piotrkluba]

Note

Medium Risk
Touches core results discovery/download plumbing and changes how several existing named-result models are populated, which could affect downloading or indexing of results if filename patterns or lazy population differ.

Overview
Adds a generic NamedResultsCollectionModel for result collections discoverable via filename regex, and migrates MonitorsResultModel, UserDefinedDynamicsResultModel, and CustomForceResultModel to use it.

Introduces ForceDistributionsResultModel + CustomForceDistributionResultCSVModel to load/download per-output *_forceDistribution.csv results (incremental or cumulative, auto-detected by headers), wires it into CaseResults.download(...), and adds SimulationParams.has_force_distributions() / Case.has_force_distributions() gating.

Extends ForceDistributionOutput validation to reject reserved names (X_slicing, Y_slicing), adds new force-distribution field constants, and updates test fixtures/mocks and validators to cover the new result files and name restriction.

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