fix: admit keyboard input to sheet fields of the observed window

[iamnbutler]

Adds peekaboo-sheet-focus.patch for githubnext/ace#390: `desktop_insert` into a Save sheet's filename field refused with "Literal insertion could not establish its read-only preflight … focused element AXWindow failed with AX error -25212" ("Exact focused-element receipt is stale"). `desktop_type` (an AX value write) worked after #8.

Cause. #8 routed the value/focus identity read through `AXWindowIDResolver.owningWindowID`, but keyboard input (literal insertion, paste, exact-window keys) validates a separate receipt in `DetachedExactWindowFocusReader.read(expected:phase:)`. That path read the receiver's own `AXWindow` attribute and required it to name the observed window. An Open/Save panel field has no AXWindow (-25212 is kAXErrorNoValue), and an in-process sheet control's AXWindow names the sheet, so the receipt was always stale. Behind that, the key-window gate required the app's focused window to be the observed window, but while a sheet is up the sheet is key, so it would refuse next.

Change.

• `AXWindowIDResolver.owner(of:)` returns the owning window and the nearest `AXSheet` crossed on the way. A missing or non-window AXWindow link means "walk the parents", not failure. `owningWindowID` is now a wrapper for it.
• The exact receipt reader and the focused snapshot use that owner (`sheetWindowID` on `ExactWindowFocusSnapshot`).
• `ExactKeyWindowSnapshot.isKey` admits a receiver inside a sheet only while that exact sheet is the focused window. Any other receiver still requires the observed window. It's used by `requireExactWindowKeyboardFocus` and the background focused-text path. A parent with an attached sheet, a different or nested sheet, sibling windows and other processes remain refused.
• Observation focus corroboration (`DetachedAXObservationWorker.focusedElement(_:belongsTo:)`) used a direct AXWindow comparison. It now uses the same resolver.
• Tests: resolver owner/sheet cases (including the no-AXWindow panel field and nested sheets), `validateOwningWindow` with the new contract, `isKey` cases, and two keyboard-dispatch tests through `requireExactWindowKeyboardFocus` (eligible when the receiver's sheet is key, and refused for another sheet, the parent holding the sheet, or the parent alone). They all use injected fakes.

Verification. `npm run check`. Full native build via `node dist/build.js --out … --scratch …` from a pristine Peekaboo checkout, applying the whole stack. `swift test` in PeekabooAutomationKit for `AXWindowIDResolverTests|DetachedExactWindowFocus*|UIAutomationExactWindowFocusTests|HotkeyServiceBackgroundPreparationTests|DetachedAXObservationWorker*|BackgroundInputDriver*`: all pass except `BackgroundInputDriverPositionalTargetTests` "positional value policy…" (focus/allowed), which fails on the AXFocused readback added by `peekaboo-point-focus.patch` and is unrelated to this diff. As in #8, the test target needed a local, uncommitted `default` case in `ClipboardPasteTransactionGateTests` to compile.

Not verified against a live Save sheet. The Mac was running a study and no GUI app could be opened. It assumes the app's AXFocusedWindow is the sheet while a sheet field has focus, which is the AppKit key-window behavior. If macOS instead reports the parent window, insertion still refuses before input ("keyboard focus is not on the target window") rather than typing.