proven_c_lib-v0.5.0

A MINOR release: nothing public removed, no behaviour changed. Three additions requested by
Pnakotic (B-044): one-character UTF-8 decoding, hexadecimal scanning, and a stated ceiling on the
error-code space. The repository now carries only the library, its tests, the manual and the
public documents; design records, benchmark archives and process notes are kept outside it, and
the public history was rewritten to match.

Added

• One-character UTF-8 decoding: proven_utf8_decode_next(s, pos). For code that walks text a
  character at a time (requested by Pnakotic, which carried a stand-in). Same strict rules as the
  rest of utf.h; the returned len is always the step to take - the character, the maximal
  subpart of malformed input (where Unicode says to resynchronise), or the bytes left when the
  text ends mid-character (NEED_MORE). Nothing is substituted. Freestanding-available.
• Hexadecimal scanning: proven_scan_u64_hex, proven_scan_i64_hex, and {:x} / {:X} in a
  scan format for any integer argument, mirroring the formatter's spec (requested by Pnakotic for
  0041..005A-style ranges). 0x is taken only before a digit, as strtoul takes it; overflow,
  cursor restore and the stream signal behave as for decimal, including a 0x that arrives before
  its digits. Before, every format placeholder had to be {}.
• The error-code space is a promise: PROVEN_ERR_RESERVED_END (0x1000) and PROVEN_ERR_LAST.
  proven will never define a code at or above 0x1000, so a program can carry proven_err_t values
  unchanged in a wider type and number its own codes from there (requested by Pnakotic).
  PROVEN_ERR_LAST names today's highest code; a build-time assertion holds the ceiling and a
  source contract keeps the macro on the enum's real last value. Aliases XCV_ERR_LAST,
  XCV_ERR_RESERVED_END, and the missing XCV_ERR_INVALID_FORMAT.

Changed

• The repository history was rewritten to drop the design records, benchmark archives, process
  notes and private verification scripts that earlier commits carried under docs/ and
  scripts/. Every commit hash changed, and every tag was moved to its rewritten commit with the
  same library sources; a checkout that pinned an old hash should re-pin by tag. Public documents
  cite the moved records by label only (RFC-0007, B-043).

Fixed

• Chapter 1 listed proven_err_t without PROVEN_ERR_NEED_MORE, in both the enum listing and
  the meaning table (English and Korean). Both now carry it.