docs(adr): ADR 009 incremental evaluation (proposed) (improvise-felo.4)

An in-house DBSP core as a problem-agnostic crate, plugged in behind a
trait in improvise-core, with the eager evaluator kept as the oracle.
Adds examples/retraction_drift.rs, which measures f64 retraction drift
against the eager fold and the exact decimal sum.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

scripts/build-doc-site.mjs

@@ -46,6 +46,7 @@ const SECTIONS = [
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       ["context/adr/004-faces-and-themes.md", "adr/004-faces-and-themes", "ADR 004 — Faces"],
       ["context/adr/007-stable-identities.md", "adr/007-stable-identities", "ADR 007 — Stable identities"],
+      ["context/adr/009-incremental-evaluation.md", "adr/009-incremental-evaluation", "ADR 009 — Incremental evaluation (proposed)"],
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