feat: visible caret and jump to start/end on the on-screen keyboard

[nextestudios]

Summary

Builds on the existing caret support in VirtualKeyboard.

• Jump to start/end: VirtualKeyboard.MoveCaretToStart/End. While the keyboard is open, LT/RT (PageUp/PageDown) call them, and so do physical Home/End (and PgUp/PgDn). Text, page and Shift state are left alone.
• Visible caret: the text field draws a steady bar (┃) in the accent color between two text runs, with no animation. It replaces the thin plain │ that was spliced into the text.
• Narrator: the field's accessible name now includes the caret ("cursor no início / no fim / na posição X de Y"). A move that changes only the cursor (text unchanged) raises a UIA notification, so Narrator speaks it without chattering on every key.
• Action bar: the keyboard context adds Início (LT) and Fim (RT) through the existing prompt mechanism (feat: add dynamic controller button prompts to action bar #34).
• LB/RB and ◀ ▶ repeat when held (enhancement: hold-to-repeat for Backspace and cursor keys in the virtual keyboard #42); LT/RT are already repeatable navigation, and repeating them does nothing extra.

Tests

CI (Windows), one new unit test:

• On a 209-character text, LT jumps to 0 and RT jumps to the end. Moving the cursor keeps Shift (Once) and the symbols page. Typing - with D-pad + South after Control gives Control-FS… with the caret after the insert. This covers both acceptance criteria.

Not verifiable in CI, so added to docs/TESTING.md:

• The caret is legible at TV distance and in 4K.
• Narrator actually speaks the caret position. The notification event and accessible name are in place, but only a manual Narrator run can confirm them. For that reason this PR uses Refs instead of Closes. Please close enhancement: improve text cursor navigation (visible caret, jump to start/end) #43 after the manual check.

Refs #43

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