fix(tui): preserve legacy editor undo

[me2seeks]

Summary

Preserve the editor's legacy Ctrl+- undo path until a side conversation exists.

The TUI now reserves the ambiguous 0x1f byte for side switching only while a side pair is active. Enhanced keyboard-protocol shortcuts and active side switching keep their existing behavior.

Fixes#3928

Verification

• Regression test fails on origin/main and passes with this fix
• npm run lint
• npm run format:check
• npm run build
• npm run typecheck
• npm --workspace maka-agent run test:dist — 514 passed
• npm run check:asf-headers
• npx knip --workspace apps/desktop
• npx knip --workspace packages/ui
• git diff --check

Additional check: npx knip --workspace packages/cli reports five unused exported types that are already present on origin/main; this PR adds none of them.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex reproduced the legacy key collision, implemented the state-scoped interception, added the regression, and performed first-principles, adversarial, and simplification reviews.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No

[jackwener]

I reviewed exact head b9efb36aec2383f5b83faa799968b0da341202ed and found no blocking issues.

The raw 0x1f ambiguity is now consumed only while a side-conversation pair exists. Before that, the byte reaches the editor's legacy undo binding; after a pair exists, both the legacy byte and enhanced Ctrl+/ sequence retain side switching. The new regression exercises the real TUI input path and confirms that undo clears the draft without switching Sessions.

The exact-head test and windows_recovery checks succeeded. A synthetic merge with current main (a6866d05918bd4c2ea25ccd682edc6033726f782) is clean. No P0-P3 findings.

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