Complete release-candidate migration testing

[Pectics]

Context

Roadmap item imported from PLAN_REQUEST.md and PLAN_RESPONSE.md for v1.0.0 — Stabilization.

Upstream references

• spencerwooo/mihoro

Upstream decision

• Ported
• Reimplemented
• Deferred
• Rejected

Reason: Reimplemented as Mihoto's v1 release gate.

Scope

• Define v0.9.0-rc.1 stabilization process.
• Run migration, rollback, scheduler, service backend, TUN, DNS, and source failure tests.
• Accept only bug, security, and documentation changes after RC unless scope is reset.

Acceptance criteria

• The scope above is implemented or documented in issue-linked PRs.
• Relevant upstream references are reviewed before implementation starts.
• Tests are added, or the PR explicitly explains why this is documentation-only.
• Documentation is updated, or the PR explicitly explains why no docs changed.
• Migration impact, security impact, and rollback behavior are evaluated.

Out of scope

• Work explicitly listed outside v1.0 in the roadmap remains deferred unless this issue is in v1.1+ — Post-v1 Backlog.
• No GitHub Project board, branch protection change, local commit, or repository file edit is part of this import pass.

Dependencies

• Depends on Publish Mihoro migration and rollback guide #28
• Depends on Publish supported distributions and systemd compatibility matrix #29
• Depends on Add network interruption, invalid subscription and rollback tests #25
• Depends on Add concurrent-update locking and crash recovery tests #24
• Depends on Pin CI actions and add dependency/license auditing #26
• Depends on Document threat model and privilege boundaries #27

Security and rollback considerations

Apply Mihoto's safety rule for this issue: updates, migrations, service changes, TUN/DNS changes, and credential handling must be explicit, validated, auditable, and recoverable. Secrets and subscription URLs must be redacted from logs and issue artifacts.