chore(licence): migrate Cargo.toml from PMPL-1.0-or-later to MPL-2.0 (#76)

## Summary
- Updates `Cargo.toml` license field from `PMPL-1.0-or-later` to
`MPL-2.0`.
- Closes the SPDX-vs-manifest mismatch flagged by
[standards#196](hyperpolymath/standards#196)
(estate licence-debt audit, 2026-05-26).
- Aligns this repo with the estate policy of `MPL-1.0/PMPL-1.0 →
MPL-2.0` migration.

## What this PR does NOT do
This is the manifest field only. The LICENSE file (if it carries a
Palimpsest preamble) keeps that preamble — the load-bearing MPL-2.0 text
is unchanged.

## Companion
- standards#196 — licence debt audit
- standards#201 — licence-consistency CI check (will catch future drift)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hyperpolymath

Cargo.toml

@@ -1,4 +1,4 @@
-# SPDX-License-Identifier: PMPL-1.0-or-later
+# SPDX-License-Identifier: MPL-2.0
 # SPDX-FileCopyrightText: 2025 Jonathan D.A. Jewell
 
 [package]