fix: the TOML reader refuses the \u, \U, \b and \f escapes of a basic string #865

Reproduction

[package]
name        = "hello"
version     = "0.1.0"
description = "café"

$ mcpp build
error: /path/to/hello/mcpp.toml:4:20: error: invalid escape '\u'

The same happens for \U0001F600, \b and \f. All four are valid escapes in a TOML 1.0 basic string, and so the manifest above is valid TOML.

mcpp.libs.toml (modules/libs/src/toml.cppm) accepts only \n, \t, \r, \", \\ and \0. The same switch is duplicated in read_string and read_multiline_string, so both single-line and multi-line basic strings and quoted keys refuse these escapes. The parser reads mcpp.toml, mcpp.lock, the configuration files and template metadata, so any of them written with a Unicode escape (often by a tool that ASCII-escapes non-ASCII text) cannot be loaded.

Expected

The parser reads every escape TOML 1.0 defines: \b, \t, \n, \f, \r, \", \\, \uXXXX and \UXXXXXXXX. A Unicode escape gives UTF-8 bytes. An escape that does not name a Unicode scalar value (a surrogate, a value above U+10FFFF, or too few hex digits) is refused with a message that names it.

Actual

invalid escape '\u' (or '\U', '\b', '\f') and the file is not read.

Environment

• mcpp version: 2026.10.29.1 (main at db3b815)
• OS: any; the parser is host-independent