docs: Rust rewrite assessment + forecast parity spike

spikes/rust-forecast ports build_features (P.1 path) and HistGradientBoosting
inference to Rust and checks them against the pandas/sklearn output on two
fixtures (2024 record flood, summer 2026): 244,800 feature cells and every
regression prediction bit-identical, classifier heads to the last ulp;
0.25 ms vs 158 ms per station. docs/RUST_REWRITE.md: what a rewrite would
mean (Rust service, Python training), costs, phased plan, Python alternative.

spikes/rust-forecast/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "ping-forecast-spike"
+version = "0.1.0"
+edition = "2024"
+publish = false
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+# float_roundtrip: exact float parsing. Tree thresholds often equal real data
+# values, so a 1-ulp parse error flips `x <= threshold` (default parser is not exact).
+serde_json = { version = "1", features = ["float_roundtrip"] }
+chrono = { version = "0.4", default-features = false, features = ["std"] }
+
+[profile.release]
+debug = false