Cut release/v11 branch from current main

[ChrisonSimtian]

First concrete step of milestone #13. Driven by RFC #267.

What

Create a new branch release/v11 from the current main HEAD. From this point onward, release/v11 is the home of all v11.x releases; main continues to receive v11 work (cherry-picked back when needed) and is also where v12-prep starts.

Acceptance criteria

• release/v11 exists on origin, pointing at the chosen commit on main.
• No commits land directly on release/v11 yet — that comes with the cherry-pick / hotfix flow.
• CHANGELOG entry under [Unreleased] — 11.0 → ### Process noting the cut and the new flow.

Notes

• The cut commit should be the latest stable main HEAD at execution time. No need to pick a special commit — main is currently the source of truth.
• The stopgap (ci: stopgap — switch release workflow to workflow_dispatch only #268) already disabled auto-releases on main, so this cut doesn't race with an auto-publish.
• Branch protection lands as a separate sibling issue.

Depends on

• Stopgap (ci: stopgap — switch release workflow to workflow_dispatch only #268) is merged ✓.

Blocks

• Branch protection on release/v11.
• main's version.json bump to 12.0.
• Refactor release.yml to tag-triggered.

[ChrisonSimtian]

Done. release/v11 cut from main HEAD.

What landed

• Branch: release/v11
• Commit: 4b19a8c1 (= main HEAD at cut time = the stopgap merge #268)
• Created via: gh api -X POST repos/.../git/refs ref=refs/heads/release/v11 sha=4b19a8c1...
• No local-worktree disturbance: branch created server-side. Chris's in-progress work on feat/fallout-core-88 is untouched.

Open PR survey

Only one open PR at cut time: #222 by @dennisdoomen (target/v11, fixes GitVersion 6.x numeric field handling). Not retargeted — per Chris's standing preference, Dennis manages his own PRs. He'll retarget if/when he wants to.

No Chris-authored PRs are currently open, so the "rebase ours onto release/v11" part of this issue is a no-op.

Verification

• gh api repos/ChrisonSimtian/Fallout/branches/release/v11 → branch exists, sha 4b19a8c1..., protected: false
• Both main and release/v11 are identical at this instant (same SHA).

What's still gated (next sub-issues)

• #270 — branch protection on release/v11 (now unblocked)
• #271 — bump main's version.json to 12.0 (now unblocked)
• #275 — validate NB.GV on release branches (now partially unblocked; full unblock after Bump main's version.json to 12.0 (next-major trunk) #271)
• #274 — refactor release.yml (still gated on env setup Create three GitHub Environments: nuget-org, github-packages, github-releases #272/Move NUGET_API_KEY from repo secret to nuget-org environment secret #273)

CHANGELOG entry deferred to the umbrella docs PR (#276) so the v11 restructure gets one consolidated entry rather than nine fragmented ones.