[textinput] Support UTF-8 at the prompt

[guitargeek]

textinput assumed one line-buffer element is one byte and one terminal column. Both assumptions break on UTF-8, so anything outside ASCII corrupted the buffer on input and misplaced the cursor on output.

Text now stores std::u32string, so every index in the library counts characters; the editor's existing index arithmetic (cursor motions, cut/paste, ...) becomes correct unchanged. GetText() still returns UTF-8, cached with a character-to-byte offset table, so Getline's C interface, the interpreter and the history file are unaffected. Since a character occupies 0, 1 or 2 terminal columns, the display now lays out lines by accumulated character width (CharWidth() in the new UTF8.h) instead of index arithmetic, and moves a double-width character that would straddle the margin to the next line.

The readers now decode multi-byte input: StreamReaderUnix assembles UTF-8 sequences (previously bytes >= 0x80 went negative, colliding with the EOF sentinel and hitting undefined isprint()); StreamReaderWin reads through ReadConsoleInputW, combining surrogate pairs and keeping the IME, dead-key and AltGr events it used to drop, and TerminalDisplayWin writes through WriteConsoleW. Also fixed: byte-vs-character index mixups in HandleControl(), Getline_color.cxx and tab completion (whose byte-space cursor could end up past the end of the line and abort), a pre-existing wrong-variable bug in the colorizer's trailing-space trim, and stale screen contents when deleting a combining mark.

Verified on Linux with unit tests for the encoding/width layer and by driving root.exe through a pty: painting, cursor columns, wrapping, editing, history, reverse search and tab completion over accented, combining, CJK and emoji input. The Windows sources are only type-checked, not built.

🤖 Done with the help of AI.

Fixes ROOT-9562

[ferdymercury]

Cool! And would fix https://its.cern.ch/jira/browse/ROOT-9562

Maybe also https://root-forum.cern.ch/t/c-20-std-format-fails/ ?

[dpiparo]

this is remarkable.

[github-actions]

Test Results

    23 files      23 suites   3d 18h 22m 38s ⏱️
 3 859 tests  3 858 ✅ 0 💤 1 ❌
79 522 runs  79 520 ✅ 1 💤 1 ❌

For more details on these failures, see this check.

Results for commit 7c5883e.

♻️ This comment has been updated with latest results.

[wacfrr]

🎉 Great! Now we are one step closer to being able to count past 255. 🚀
🎉 太好了！距离我们能数到255以上又近了一步。🚀

[guitargeek]

1000 lines added in the component every interactive root session goes through, but AI-assisted. The pty-driven testing described and local tests are reassuring, but this is the kind of change you want to land early in a dev cycle so it soaks on master and get some testing from developers and power users, not right before a release branch is cut (which will happen in November).