feat(a11y): add core keyboard and browser contracts

[mattheliu]

Summary

• Build the accessibility-core candidate from exact dsh-v0.1.2-alpha.2 and migrate behavior by interaction owner.
• Make shared Modal own inertness, focus containment/restoration, nested topmost dismissal and descriptions; move Settings and Workspace error paths onto that owner.
• Give grouped, flat and search Workspace views a single roving tree entry with arrows, Home/End, disclosure, activation, typeahead and row-action focus ownership.
• Require persistent-shell sends to return to an exact stdin wait; accept a sampled leave-and-return or post-submission output for fast transitions, while rejecting a pre-write wait or same-shell echo plus silence alone.
• Add a required assembled-app Chromium/Firefox/WebKit accessibility job on standard hosted fork runners.

Evidence

• Exact candidate: dc9cc8d6145a7007c8f23a987234daa7f2442d09.
• Workspace tree owner: 52/52 tests; 100% statements (91/91), branches (86/86), functions (11/11), lines (80/80).
• Terminal session owner: 54/54 tests; 100% statements (452/452), branches (275/275), functions (74/74), lines (394/394). The local real-PowerShell composition test is skipped because this macOS host has no pwsh; that skip is not counted as PowerShell evidence.
• Built assembled app: 2/2 paths passed independently in Chromium, Firefox and WebKit.
• Full Web snapshot replay: 89 files passed, 1 capability skip; 303 tests passed, 15 capability skips.
• Recorded-session snapshots in the CI lib mode: 114 passed, 2 capability skips.
• CI static aggregate: 43/43 gates passed. SQLite schema owner: 86/86 tests passed. Full build and pre-push typecheck passed.
• Failed history is preserved without rerun. Run 33330458425 exposed the missing exported-hook JSDoc, a nested npm browser invocation, stale accessibility expectations, a fast persistent-PowerShell transition that escaped polling, and a 100 ms SQLite retry fixture budget under Windows coverage. This head contains deterministic corrections for each observed cause.
• New attempt-1 main CI: https://github.com/omdsh-dev/deepseek-harness/actions/runs/33332020842

Scope and limitations

This is an alpha.2 migration checkpoint, not a claim that all of DSH is accessible. Automated DOM and browser contracts do not substitute for task-level VoiceOver, NVDA, other assistive-technology sessions, braille/input evidence, or disabled-developer validation. Menus, shell landmarks and separators, conversation/tool/trajectory flows, announcements, contrast, reflow, motion and remaining core-task protocols are still open. No merge, release tag or npm publication is proposed.

Related to omdsh-dev/dsh-accessibility#22.