Plan the artist platform: price calculator and fan-funded pressing

New docs/artist-platform.md: upload master and artwork with automatic
audio, artwork and rights checks; an itemised price calculator for every
customer (per-title vs per-copy lines, price-per-copy curve, lead time
from the production-timeline quote table, versioned price list); and
fan-funded campaigns that press and dropship per fan once the goal is
reached, refund otherwise. Covers goal maths, payments (charge into a
ring-fenced account, CNX as seller of record, legal opinion first; the
Qrates failure as the design rule), repress pre-orders, architecture on
the cnx-factory stack with the public side on a fleet machine, PDPA,
phasing (calculator public early as a run-size probe, campaigns only
after first releases ship), cost and open decisions.

Backlinks from README, process flow, production timeline, revenue model,
digital factory and roadmap items 1 and 20; total investment added-scope
table gains the platform line.

docs/process-flow.md

@@ -11,6 +11,11 @@ A printable diagram of this flow lives alongside this document:
 [process-flow.png](process-flow.png), and
 [process-flow.pdf](process-flow.pdf).
 
+Orders reach stage 1 either directly or through the
+[artist platform](artist-platform.md), where the upload, the automatic
+audio and artwork checks and gate 1 happen before a fan-funded campaign
+is allowed to go live.
+
 ```mermaid
 flowchart TD
     A[1. Audio intake] --> G1{Artist gate:<br/>master plan approved}