fix(tui): live dictation can panic when the caret is not at the end

[PierrunoYT]

Version / branch / commit

Audited on main at 903e3d13 (2026-08-23).

Problem

Live dictation anchors its region on the full composer text but later compares and slices only the prefix before regionStart. When the caret is inside existing text, the external-delete adjustment can make regionStart negative. The next partial transcript slices []rune(text)[:regionStart] and panics.

Reproduction

1. Put hello world in the composer.
2. Move the caret to column 5, after hello.
3. Start live dictation and receive successive partial updates.

The first update can compute anchor = "hello world", prefix = "hello", and delta = 6, moving regionStart from 5 to -1. A subsequent partial slices with the negative index.

Expected behavior

Dictation updates remain bounded to a valid composer range regardless of caret position or concurrent typing/deletion.

Actual behavior

A negative slice index panics the TUI process and can lose the in-flight run.

Relevant code

• internal/tui/dictation_stream.go:103,115,120-132

This is related to #936's voice-mode/typing collision but distinct: #936 concerns swallowed spaces and stray transcription, while this issue is a negative-index process crash.

Suggested fix

Clamp and validate regionStart and regionEnd before every read and slice, then add a regression test with the caret in the middle of existing text and at least two partial updates.

[PierrunoYT]

I'm working on this.