[Stage 0.5] Perform the controlled v1/v2 branch split

[draedful]

Parent: #310

Goal

Create the independent v1 maintenance line and make main the v2 development line without an unprotected release window.

Scope

• Use the split commit approved in Stage 0.1.
• Create release/v1 from that exact commit.
• Keep main as the default branch.
• Confirm that v1 release pull requests and release commits target release/v1.
• Prevent main from publishing a v1 version.
• Initialize the agreed v2 development or prerelease state on main.
• Confirm that no workflow automatically merges, rebases, or synchronizes the two branches.

Acceptance criteria

• release/v1 points to the approved split commit.
• main remains the default branch and is explicitly treated as the v2 line.
• v1 release automation targets only release/v1.
• main cannot produce a 1.x release.
• Both branches are covered by the required rulesets.
• There is no automatic cross-branch synchronization.