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

docs/TODO.md

@@ -282,5 +282,12 @@ adoption the obvious choice.
   whole headless path. Verified dependency-free (no bare imports in the output)
   and end-to-end (a smoke test composes a decision from the entry). Documented
   in the README Packages table and the headless section; wired into `npm run build`.
-- A worked example of BitGate feeding a feed ranking (the `ranking` dimension →
-  a score multiplier) would have saved us reinventing it in the feed recipe.
+- **[DONE 2026-08-11]** A worked example of BitGate feeding a feed ranking.
+  **Shipped** `examples/feed-ranking.mjs` (runnable): it reads `decision.ranking`
+  (`{ effect: "normal" | "downrank", weight }`) and turns it into a score
+  multiplier (`base ** weight` for a downrank), so trusted spam reports push two
+  high-base authors below a clean lower-base one **without hiding them** —
+  visibility stays a separate dimension, capped at `restrict` on a feed. The
+  multiplier lives in the example, not the engine (core ships no such numbers).
+  Pinned by `tests/feed-ranking.test.js`; the README headless section points to
+  it.