Plotting improvements — autoscale README example and plot utility wrappers

[danmcgraw94]

Short-term (README fix)

The README's plotting example has hard-coded y-axis limits tuned for JMD. For any other dam, users have to manually edit the code. Switch to autoscaling (e.g., using range() on the data or letting ggplot default) so the example works generically.

Define a package-level ggplot2 theme that establishes the visual identity for all rfaR figures.

• Typography: consistent font family, sizes for title/axis/legend.
• Panel and grid styling appropriate for frequency analysis plots (log-probability x-axis, linear or log y-axis).
• Color palette — define a standard set of discrete and continuous scales (consider ggsci as a base).
• Legend placement and formatting defaults.
• Export-friendly: clean appearance at both screen resolution and print (300 dpi).

Longer-term (code)

Expose convenience wrappers in plotting_utils.R for standard rfaR plots (stage-frequency curve, conceptual realization) with sensible defaults and optional arguments for overrides like y-axis limits. Follows the existing theme_rfar_conceptual() pattern.

Function	Output Type	Key Elements
plot_stage_frequency()	Stage-frequency curve	AEP x-axis (probability scale), stage y-axis, median + uncertainty bounds
plot_hydrograph()	Routed or input hydrograph	Time x-axis, flow/stage y-axis, support for overlaying multiple hydrographs
plot_routing_trace()	Modified Puls routing result	Inflow vs. outflow vs. pool stage over time
plot_frequency_fit()	LP3/GEV distribution fit	Empirical plotting positions + fitted curve, optionally with MCMC/bootstrap envelope
plot_starting_pool()	Starting pool distribution	Histogram or density of sampled starting pool elevations

Feedback source

Raised by Allen (comments #5, #8).