plot: add stacked series composition

[imakris]

Implements a caller-visible stacked-series slice:

• .stack_group(id) opt-in on ordinary series
• independent source LOD/window selection
• timestamp-union interpolation within the common valid domain
• cumulative LINE/DOTS and AREA base/top bands
• a generic renderer-owned Σ total overlay for every successfully composed stack: the final cumulative geometry is drawn above component primitives through the existing LINE pipeline, with a line width of Plot_config::line_width_px + 2px
• exact overlay theme colors: dark mode #E6DFCC, light mode RGB inverse #192033; component fills, lines, and dots retain their own colors
• total overlays in both main and preview views, including AREA-only and DOTS-only top components; singleton and rejected groups remain overlay-free
• main/preview/auto-range/Qt flow
• composition cache plus input/output/cache-hit observations
• fail-closed behavior for incompatible stack inputs
• global Stack functions toggle in the function plotter; existing and newly added functions share one stack while enabled
• drag reordering from each function's color accent or f(xN) = label; the model and plot IDs update so the visible order is the actual stack order
• native Qt SpinBox stepping, so repeated sample +/- clicks update both the displayed count and plotted samples after typed input
• fractional function-domain x values in the data indicator, while axis tick formatting remains unchanged
• raw component indicator values plus cumulative markers in plot-ID stack order and a textual Σ total; the indicator notes that markers show cumulative stack positions
• marker geometry is renderer-gated and fail-closed for unresolved multi-member stacks, while a lone nonzero stack_group member keeps its ordinary raw marker
• generic Plot_widget::auto_adjust_view / Fit Vertical fitting of current renderer-composed cumulative stacks, including AREA bases; stale or rejected stacks fail closed, and renderer-ineligible hidden peers do not suppress or contaminate singleton fitting
• a width-neutral wrapped cumulative-marker note in the reusable PlotIndicator; x/value rows alone determine indicator width and wrapped note lines add height only

Correctness checks cover differing timestamps, independent LODs, overlap clipping, cache reuse/invalidation, rejected-stack suppression, auto-range, API propagation, function reordering, fractional indicator x formatting, raw component text, cumulative marker positions, note visibility and wrapping, renderer rejection/staleness, common-domain clipping, singleton marker behavior, stacked Fit Vertical envelopes, ordinary unstacked fitting, and hidden-peer renderer eligibility. Focused renderer checks additionally cover overlay presence and ordering, final cumulative geometry reuse, exact dark/light colors, thickness, multiple groups, singleton/rejected exclusion, main/preview rendering, AREA-only and DOTS-only tops, component-color preservation, and reuse of a top LINE component's existing line-window upload.

A native QRhi smoke renders two stacked AREA series with offset timestamps and validates composition, nonuniform readback, and pixels in the exact dark-mode overlay color #E6DFCC; the ordinary smoke remains unchanged. FunctionPlotterControls also verifies repeated sample stepping after typed input.

Independent review found one generic Fit Vertical eligibility blocker: a style-NONE/no-MAIN-layer peer could be counted as a stack member although the renderer skipped it. The fix now uses the renderer's existing MAIN eligibility API (qrhi_layers_for and draws_view) and a focused regression proves the visible singleton retains ordinary fitting. The bounded review of the total-overlay addition reported no blockers.

The text-enabled Release build and all 30 configured CTests pass, including both native QRhi smokes and the stacked AREA exact-color smoke. Task-touched files received canonical formatting; generic QML lint, diff checks, focused post-review tests, and a rebuilt function-plotter runtime load smoke also pass.

The isolated product review target is recovery/remove-process-damage..feature/stacked-series. This PR temporarily targets master so existing cross-platform workflows run; it depends on cleanup PR #21.