RELEASE.md: multi-extension distribution versioning procedure

[jnasbyupgrade]

Summary

• Documents the versioning procedure for a distribution that provides more than one extension (test_factory is the first repo in the org to actually need this: test_factory + test_factory_pgtap).
• Key rule: the distribution version always advances at every release; each extension's own version only moves if its own sql/<ext>--<last-released>--stable.sql update script has real content by release time -- and when it does move, it moves to the same value as the new distribution version, not an independently-numbered scheme.
• Requires maintaining a stable-target update script for every extension a distribution provides (not just the one(s) a given PR touches), so there's something to inspect per-extension at release time.
• An untouched extension gets its default_version reverted to its own last real version for the tagged archive (never left at the literal stable), and its no-op update script gets excluded from the release archive via .gitattributes export-ignore.
• Fixed a stale "step 7" cross-reference that should have said "step 8" (predates this change, caught while touching the same paragraph).
• CLAUDE.md: documented a gotcha caught while implementing this on test_factory -- the stable pseudo-version's gitignore entry must name sql/<ext>--stable.sql exactly, not a sql/*--stable.sql glob (which also matches across the update script's own second -- and silently sweeps up a file that must stay committed).

Test plan

This is a documentation-only change. Verified against a real implementation on test_factory (Postgres-Extensions/test_factory#42): the export-ignore glob behavior, the no-op update script requirement (ALTER EXTENSION ... UPDATE genuinely fails without one, even for a no-op transition), and the gitignore glob-ambiguity gotcha were all confirmed directly there before writing this up.

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