feat(input): add password input support

[LeTamanoir]

Adds type: "password" support to the existing <input> component, masking
typed characters without any changes to the native rendering layer.

What's new

• type option - "text" (default) or "password"
• maskChar option - character used for masking, default "*", must be
  exactly 1 character; configurable at construction time or at runtime
• Both options have getters/setters and trigger a re-render on change

Behaviour in password mode

• Typed characters are displayed as maskChar; input.value always returns
  the real text
• All events (INPUT, CHANGE, ENTER) emit the real value, not the masked
  one
• getSelectedText() returns "" - clipboard-safe by default
• Word-boundary navigation (Ctrl+←/→) jumps to absolute start/end instead
  of word boundaries
• Word deletion (Ctrl+Backspace/Delete) deletes to start/end of the field
• Placeholder renders normally when the field is empty
• Horizontal scrolling works correctly on long passwords

Usage

// Renderable API
const input = new InputRenderable(renderer, {
  type: "password",
  maskChar: "●",   // optional, defaults to "*"
  placeholder: "Enter password…",
})
// React / Solid - props surface automatically
<Input type="password" maskChar="•" />

[LeTamanoir]

Hey @simonklee, quick question on type: "password" support!

In password mode every character renders as *, which gets weird with wide characters. A 🎍 takes 2 columns in the buffer but only 1 as *, so a width: 5 input starts scrolling after just 2 emoji. Same issue with maxLength.

Should width/maxLength count graphemes (like HTML <input type="password"> does) or display columns of the real text (so emoji "cost double")?

Any preference or prior art you've seen? Happy to go either way!