docs: worked example of the ranking dimension as a feed multiplier

The `ranking` dimension is the soft one — "still show this, but lower" — and
integrators reinvented turning it into a feed score. Show it instead.

Add examples/feed-ranking.mjs (runnable): it reads decision.ranking
({ effect: "normal" | "downrank", weight }) and turns it into a multiplier
on the app's own feed score (base ** weight for a downrank). Two authors
that led on base score get downranked by trusted spam reports and drop below
a clean lower-base author -- without being hidden, since visibility is a
separate dimension (capped at "restrict" on a feed). The multiplier lives in
the example, not the engine: core ships no such numbers on purpose.

Pinned by tests/feed-ranking.test.js (downrank never hides on a feed; a
high-base downranked author reorders below a clean one; a clean author is
untouched at x1). README headless section points to it.

Addresses the "BitGate feeding a feed ranking" item of the Tessera feedback
(docs/TODO.md) -- the last open BitGate item.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01BXQCvJPA6sVSQ5j8uBd9bB

README.md

@@ -161,6 +161,12 @@ No bundler? `npm run build:standalone` produces `packages/standalone/dist/bitgat
 ESM file. Self-host it and `import { evaluateTarget, decodeLabels } from "./bitgate.js"`;
 everything in this section works from that single file.
 
+The soft dimension, `ranking`, feeds an ordering rather than a gate:
+`decision.ranking` is `{ effect: "normal" | "downrank", weight }`, which an app
+turns into a multiplier on its own feed score — a reported author drops in the
+feed without being hidden. [`examples/feed-ranking.mjs`](examples/feed-ranking.mjs)
+is a runnable walkthrough.
+
 ## Extending to a new application
 
 Write an adapter saying what your object is in governance terms, and a policy