Add searchable — a navigation search field

[colemancda]

Adds .searchable(text:prompt:), which attaches a search field to the enclosing navigation stack bound to a String. The developer filters their own content off the bound text — the standard SwiftUI idiom.

Mechanism

Follows the exact pattern navigationTitle already uses: the modifier is a _ResolutionEffectView that records its field into the per-screen TitleSink (current text, a registered string callback, prompt). NavigationStack._render collects each screen's descriptor into a parallel searches prop, and the interpreter renders an OutlinedTextField (search icon + prompt) above that screen's content.

Text round-trips with the same uncontrolled-with-reconciliation scheme as TextField — Compose owns the cursor, lastSent suppresses the echo — keyed by the stable screen index rather than the callback id (which churns every evaluation).

Verification

• swift test — new emission/round-trip test (asserts the root screen's descriptor carries text + prompt, and that string input drives the binding), 69 total passing
• Emulator (new "Searchable" catalog screen, a fruit list): the field surfaced under the nav title with a search icon and "Search fruit" prompt; typing a narrowed the list to the eight fruits containing 'a', and adding p (→ ap) narrowed it live to Apple / Apricot / Grape, with the text accumulating and the cursor preserved across each round-trip

Scope

Like alert/sheet, the field is picked up per navigation screen; searchable outside a NavigationStack is a no-op (no title sink), matching how navigationTitle behaves. Search scopes and isSearching are not included.