docs(demo): nest evidence scorer in demo profiles (#686)

## Summary

### Why?

Demo profiles still showed a sibling `predictor:` block, which the wired
YAML no longer accepts.

### What?

Use `defaults.scorer` with `type: evidence`, the RFC example factors
(`landing` for the land-stage batch), and a nested heuristic `base` in
fake/git/github profiles and the quickstart.

## Test Plan

Docs and YAML only; config parse is covered on the wiring PR.

## Stack

- #684
- #686 ⬅️

Author: behinddwalls

doc/howto/QUICKSTART.md

@@ -74,11 +74,20 @@ Every change writes all of its files into one folder under `demo/`, and `FOLDERS
 
 Set it deliberately when you want a run to show one thing. `FOLDERS=1` puts every change in the same place, so the queue serializes the lot and each change speculates on the one before it. A number well above `COUNT` keeps them all apart, so they go out together.
 
-How much speculation that turns into is capped by the queue's **build budget** — how many builds it may have occupying CI at once, counted across every in-flight batch rather than per batch. It defaults to 4 and is set per queue in the provider's `profiles.yaml`:
+How much speculation that turns into is capped by the queue's **build budget** — how many builds it may have occupying CI at once, counted across every in-flight batch rather than per batch. It defaults to 4 and is set per queue in the provider's `profiles.yaml`. The demo also sets **evidence scorer factors** there so speculation ranking revises the base price when a path passes or fails or a batch is merging or cancelling; omitting `factors` leaves every factor at `1`, which is a no-op and ranks on the nested base alone.
 
 ```yaml
 defaults:
   speculator: {buildBudget: 4}
+  scorer:
+    type: evidence
+    factors:
+      pathPassed: 10
+      pathFailed: 0.3
+      landing: 12
+      cancelling: 0.1
+    base:
+      type: heuristic
 
 queues:
   - name: demo-queue

service/submitqueue/demo/provider/fake/profiles.yaml

@@ -22,6 +22,18 @@ defaults:
   # 1 the queue explores one path at a time, and raising it lets it hedge more
   # of the outcomes it is waiting on. Four is the built-in default.
   speculator: {buildBudget: 4}
+  # Example evidence factors from the outcome-scorer RFC. Omitted keys would
+  # stay at 1 and ranking would stay on the base price; writing them out makes
+  # the demo show how path results and batch state revise that price.
+  scorer:
+    type: evidence
+    factors:
+      pathPassed: 10
+      pathFailed: 0.3
+      landing: 12
+      cancelling: 0.1
+    base:
+      type: heuristic
 
 queues:
   # The queue `make demo-requests` and `make land` use by default.

service/submitqueue/demo/provider/git/profiles.yaml

@@ -12,6 +12,17 @@ defaults:
   buildRunner: {type: fake}
   # Serialize conservatively unless a queue says otherwise.
   analyzer: {type: all}
+  # Same example factors as ../fake/profiles.yaml — see that file for why they
+  # are written out rather than left at the neutral default of 1.
+  scorer:
+    type: evidence
+    factors:
+      pathPassed: 10
+      pathFailed: 0.3
+      landing: 12
+      cancelling: 0.1
+    base:
+      type: heuristic
 
 queues:
   # The queue `make demo-requests` and `make land` use by default. Serializes

service/submitqueue/demo/provider/github/profiles.yaml

@@ -10,6 +10,17 @@ defaults:
   changeProvider: {type: fake}
   buildRunner: {type: fake}
   analyzer: {type: all}
+  # Same example factors as ../fake/profiles.yaml — see that file for why they
+  # are written out rather than left at the neutral default of 1.
+  scorer:
+    type: evidence
+    factors:
+      pathPassed: 10
+      pathFailed: 0.3
+      landing: 12
+      cancelling: 0.1
+    base:
+      type: heuristic
 
 queues:
   - name: demo-queue