Write the upgrade guide in English

[gplanchat]

UPGRADE.md, 324 lines, French from top to bottom. It is what a consumer reads when a release
breaks something — the "documentation in every case" half of this repository's own rule.

What is checked, not claimed

• The eight fenced blocks diff clean, except two comment lines. Extracted from both versions and
  compared in order: composer require, vendor/bin/rector process src, the rector.php sample,
  both bin/console cache:clear, the durable.nexus_handler: … is served by nobody output and the
  tags YAML are what a reader copies, and they are untouched.
• The two lines that did move are comments inside fences — one // in the rector.php sample,
  one # in the tags YAML — each with the code on its line and its column alignment unchanged. The
  brief named // only; the agent translated the # too and flagged it as a decision rather than
  an oversight. It was right: a French comment left inside an English guide is the visible defect
  the fence rule exists to prevent.
• Fifteen headings keep their level and order, and ## 0.1.0-alpha8 is byte-identical.
  ## Non publié becomes ## Unreleased, which is prose.
• Every backticked symbol outside a fence is identical before and after — sorted and compared as
  a multiset. That covers durable.event_store.dbal, Gplanchat\Durable\Store\EventStoreInterface,
  AsActivityHandler, MEMO_KEY_JOURNAL_BOOTSTRAP, and the \BadMethodCallException versus
  BadMethodCallException distinction, which the file makes deliberately and keeps.
• The three tables were realigned, not left ragged: English is not the width of French. Each now
  has exactly one pipe layout, and each keeps its own separator style.

After this

Nothing French is left outside documentation/ (whose *.fr.md are WA006's own exception),
.cursor/ (another tool's configuration), openspec/ (point-in-time records) and the two
supervised scripts bin/splitsh-publish.sh and bin/prises-check*.sh — the second of which prints
the French you see in the "Stale claims and titles" check.