feat: prepare TermUI v2

[mikehostetler]

Summary

• Establish next/v2 as the focused TermUI v2 work branch.
• Replace the process-heavy component model with one Elm-style runtime and pure, parent-owned widgets.
• Restore SSH support through complete backend-owned sessions.
• Close the migration gaps tracked by Plan: close TermUI v1-to-v2 migration gaps #47.

Status

Do not merge this pull request yet. It is the visible review and integration record for v2 work. The existing develop branch remains unchanged until a separate release decision is complete.

This work supersedes the closed cross-fork PR #41.

Verification

• mix quality
• mix coveralls: 965 tests, 0 failures, 90.0% coverage
• Complete next/v2 branch CI passed: https://github.com/agentjido/term_ui/actions/runs/33503327264

Branch

Head commit: 048ae09

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