[feature] Public separation between passive hint text and transient helper messages

[kzhrv]

Hi, I’m integrating reeflective/readline into a shell that has multiple “helper” concepts:

• passive hint text based on the current input
• transient forced messages/status updates
• sometimes preview-like content

Right now the single helper surface is useful, but it makes it difficult to keep those concepts distinct.

Current issue

If a shell has:

• passive hint text from the current line
• a transient message from an async event
• preview-ish content

there is no clean public separation between those channels.

That means integrations have to choose who wins, or collapse everything into one helper lane.

Requested API

Something like:

func (rl *Shell) SetHintProvider(func([]rune, int) []rune)
func (rl *Shell) SetTransientHint(text string)
func (rl *Shell) ClearTransientHint()

or any equivalent design that clearly separates:

• passive/background hinting
• explicit one-shot status messaging

Why this matters

This would improve:

• shell UX composition
• async status reporting
• preview/hint coexistence
• frontend parity for shells that already model these separately

Even if preview stays separate in a future API, having hint vs transient-message separation would already help a lot.