The serve-time decimation (#108) reduced every series by per-bucket min/max. That is
the right envelope for a cumulative equity curve but wrong for the bounded exposure
stream (C10, f64 ∈ [-1,+1]): over a multi-year window each ~hundreds-of-bars bucket
straddles sign flips, so min/max is ±1 in nearly every bucket and the exposure
collapses to a solid -1..+1 band that reads as per-point oscillation — even for a
calm strategy (a 50/200 member flips only 0.81% of bars yet still bands out).
Make decimation tap-aware (RED-first, #111): a new `ReduceKind {MinMax, Mean}` on
each Series, set by the chart builders via `reduce_for_tap` (exposure -> Mean, else
MinMax). `decimate` honours it — a Mean series emits the per-bucket mean (its
net/duty-cycle level) in both spine slots, a MinMax series keeps min/max. The shared
spine, the equity rendering, and the page payload are unchanged (reduce is
server-side only, `#[serde(skip)]`). Verified on real GER40 1y M1: the served
exposure series goes from a ±1 band to a smooth net-level line in [-0.38, +0.34].
Rendering the min/max envelope honestly (range bars / OHLC, vs today's polyline) is
the deferred other half — filed as #112.
Verified: cargo test --workspace = 447 passed / 0 failed (incl. the RED-then-GREEN
decimate_mean_reduces_a_bipolar_series_to_its_bucket_level); clippy -D warnings clean.
closes#111
refs #112
Brummel
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