Keep new-settlement confirmation keyboard focus inside its dialog

The production New settlement confirmation does not take or contain keyboard focus. In the rebuilt browser game, opening it leaves focus on the header button; pressing Tab focuses the map canvas behind the confirmation. Escape then reaches the map and leaves the confirmation open. Its accessibility node is an unnamed container rather than a named dialog.

Reproduction through actual browser input:

1. Start and pause an original tutorial settlement.
2. Click New settlement.
3. Press Tab. The focused element is the Settlement map canvas, while Start a new settlement? remains visible.
4. Press Escape. The confirmation remains visible and focus remains on the map.

Source owner: the application confirmation in resources/views/index.stx, using maintained stx/Stacks UI focus and dialog primitives. Related #90 and the shared keyboard controls. Do not add document/window access to the STX script or copy a framework focus runtime into the app.

Acceptance:

• Retain a real production-browser keyboard regression for opening, Tab/Shift+Tab, Escape and focus restoration.
• Give the confirmation its proper accessible name and dialog semantics. Focus the seed input or another appropriate initial control when it opens.
• Contain keyboard navigation and prevent underlying game shortcuts while the confirmation is open. Preserve its own input editing.
• Restore focus to New settlement when cancelled, and to the map after starting a new settlement. Retain seed validation/error behavior and existing checkpoint replacement behavior.
• Use maintained stx/Stacks UI capabilities, correcting an upstream defect in its owner if the required primitive does not deliver its documented behavior.
• Verify short desktop and narrow screens, reduced motion, lint/types and the full production browser gate. Qualify pushed CI diagnostics before closing.

The separate remembered-world correction is tracked in #222.

The confirmation now uses the maintained STX Dialog with a named title, initial seed focus, contained Tab/Shift+Tab navigation, Escape/cancel focus restoration, invalid-seed focus, and map focus after starting a new settlement. The panel fits desktop, short landscape and narrow phone viewports. Native browser input passed those flows at all three sizes, including a saved seed surviving checkpoint reload and reduced-motion/device-scale checks.

The added native outside-click check exposed an upstream component defect: DialogPanel's full-screen layout covers DialogBackdrop. The owning fix and native pointer regressions are tracked in stacksjs/stx#2124. Both centered and sheet variants now pass locally with the source correction. Publication, Stacks adoption and the production game gate remain required before this issue closes.

The confirmation now uses the maintained dialog components. It takes seed focus, contains Tab and Shift+Tab, preserves native input editing and validation, restores the opener on cancellation, and focuses the map after replacing the settlement. The upstream backdrop hit-testing defect is corrected in STX #2124, delivered through STX 0.2.719 and Stacks 0.75.328.

The new production-browser regression passed 135 steps across desktop, short landscape at 2x, and narrow screens with reduced motion. All 17 local browser suites passed 681 steps, including the unchanged ten-minute rival battle, with no recorded browser errors. Unit tests passed 426 tests; lint, types, original assets and the production build passed too.

The receipt includes original publication verification and the UI review. CI is running; this stays open until its browser diagnostics are qualified.

The settlement confirmation now has named dialog semantics, seed focus, contained keyboard navigation, native input editing and validation, Escape/cancel/outside-click dismissal, and correct focus restoration. Successful replacement focuses the map and restores the new checkpoint on reload. Short and narrow screens retain usable scrolling and visible focus; placeholder contrast and paragraph spacing are improved.

CI passed: 426 unit tests and all 17 production-browser suites, totaling 681 steps with no recorded browser errors. The original diagnostic ZIP was downloaded and checked against its API digest, size, every CRC and the exact JSON/log allowlist. The full rival battle and existing original-terrain checks were retained.

The receipt records the upstream fixes and verified STX 0.2.719 / Stacks 0.75.328 adoption. This completes the confirmation issue; broader game accessibility and original Gold qualification remain separate work.