refactor(dialog): consolidate accessibility traversal

[steipete]

Summary

• move dialog graph traversal into a dedicated owner with one shared cycle-safe, early-stoppable depth-first engine
• centralize sheet-first child ordering so each node reads accessibility children once and repeated sheet identities are queued once
• reuse the collected button snapshot during selection instead of traversing the dialog twice
• add deterministic coverage for sheet ordering, identity deduplication, and button selection policies

Behavior preserved

• depth-first pre-order and first-match short-circuiting
• direct child sheets before attached sheets before ordinary children
• title-less dialog resolution remains attached-sheets-only
• existing button title, identifier, default, and cancel fallback order

Test plan

• DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer swift test --package-path Core/PeekabooAutomationKit --filter 'Dialog(ElementTraversal|ButtonResolution)Tests' --no-parallel
• DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer swift test --package-path Core/PeekabooAutomationKit --no-parallel
• DEVELOPER_DIR=/Applications/Xcode-beta.app/Contents/Developer swiftlint lint --config .swiftlint.yml (13 existing non-serious warnings, none in changed files)
• safe release checks plus swift test --package-path Apps/CLI -Xswiftc -DPEEKABOO_SKIP_AUTOMATION --no-parallel

No changelog entry: this is an internal refactor with unchanged user-visible behavior.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: refactor(dialog): consolidate accessibility traversal This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.