docs(readme): flip stale PMPL-1.0 license badge to MPL-2.0 (#84)

Single-line README badge polish. Companion to #83 (source SPDX sweep).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -1,6 +1,6 @@
 = proof-of-work
 
-image:https://img.shields.io/badge/License-PMPL--1.0-blue.svg[License: PMPL-1.0,link="https://github.com/hyperpolymath/palimpsest-license"]
+image:https://img.shields.io/badge/License-MPL--2.0-blue.svg[License: MPL-2.0,link="https://www.mozilla.org/MPL/2.0/"]
 
 
 :toc: