canvas: carry click modifiers into press Msg arms

[MohakBajaj]

The gap

WidgetPointerEvent already carries modifiers, and the platform layer
already fills them — but nothing forwards them to app code. UiApp.Options
has on_key, on_pinch and on_drop and no pointer hook, and markup's
on-press carries a tag plus one payload path. So cmd-click and
shift-click are not expressible in either core language, in markup or in a
Zig builder view.

I hit this building a capture inbox: multi-select had to fall back to a
context-menu item, because the modifiers the runtime already knew were
dropped one layer before the app.

The change

Carry them on the existing press channel, the way on-drag already fills
phase and geometry around an authored sourceId. A Msg arm that declares
the four booleans shift / control / alt / super beside at most
one authored payload field receives them:

| { readonly kind: "focus_item"; readonly id: number;
    readonly shift: boolean; readonly control: boolean;
    readonly alt: boolean; readonly super: boolean }

<panel on-press="focus_item:{row.id}">   <!-- markup is unchanged -->

Markup fills id; the runtime fills the rest at dispatch. A bare
on-press="clear" names the modifier-only form. An arm without all four
booleans is an ordinary payload and takes exactly the path it takes today,
so this is additive — no new attribute, no new vocabulary in the closed
grammar.

What is touched

• ui_markup_reflect.zig — declaredWidgetPressRecord, pressPayloadFieldName
• ui.zig — msgForPointerClickModified + the withPressModifiers
  injector; the three-argument msgForPointerClick stays as the
  unmodified entry point
• both engines (ui_markup_view.zig, ui_markup_compiled.zig) fill the
  authored field and leave the modifiers to dispatch
• ui_markup_contract.zig — the press payload class and
  MsgTag.press_payload, so a binding is kind-checked against the
  authored field rather than the record; artifact version 6 → 7
• automation — widget-click <view> <id> [cmd|shift|cmd+shift], so the
  channel is drivable from tests and smoke runs
• docs page + native-ui / automation skills

Verification

• zig build test passes. The new dispatch test was confirmed to actually
  execute by forcing it to fail first, then reverting.
• zig build test-examples-native passes, including kanban (the
  adjacent on-drag path) and the TypeScript-core examples that exercise
  the contract.
• Driven end to end in a real app built against this branch: plain click
  clears, shift-click makes a range, shift-click again replaces it,
  cmd-click adds discontiguously, plain click clears. Standard macOS list
  selection.

scripts/gate.sh fast is green except bench-check, which fails the same
budgets worse on clean main on this machine (keystroke-big-view
3647µs vs 3567µs with the change, against a 3000µs budget) — it is not a
quiet Apple Silicon benchmark host, not a regression here.

Notes for review

• The artifact version bump is the invasive part. Every app's
  zig-out/model-contract.zon regenerates on the next build. It degrades
  loudly and never falsely passes (I watched an app report "could not be
  parsed — run native test"), but if you would rather have a different
  compatibility story, say so and I will rework it.
• clickCount is deliberately excluded. The runtime has it and it
  would be nearly free, but on-double-press already covers that ground
  and a narrower first change seemed easier to judge. Happy to add it.
• CHANGELOG.md is untouched, per AGENTS.md.

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