feat: gnu wc 9.11-compatible CLI, byte-identical output

Adds -L/--max-line-length (display width, tab stops every 8), the long
options --lines/--words/--chars/--bytes/--max-line-length, --files0-from
(NUL-delimited name lists), --total=auto|always|only|never, --debug
(acceleration diagnostics), and POSIXLY_CORRECT handling. Ports GNU's
column width rule (digits of the sum of regular file sizes, floor 7 for
non-regular inputs), its multibyte word/line-length semantics (-w/-m/-L
decode per character under a multibyte locale), the -c sizing heuristic
(page-aligned st_size reads a tail block), and its error texts, quoting
and exit codes.

Verified byte-identical stdout, exit codes and (normalized) stderr
against coreutils wc 9.11: 125-case parity suite and 552-case fuzz
across C, C.UTF-8, en_US.UTF-8 and POSIX locales. The benchmark contract
holds: words, lines (incl. the 100M monster) and stdin all pass with
fastwc never slower than GNU wc.

README.md

@@ -60,11 +60,22 @@ stopped being a challenge and started being a participation trophy.
 ## What it does
 
 ```
-fastwc [-lwc] [-m] [file...]
+fastwc [OPTION]... [FILE]...
+fastwc [OPTION]... --files0-from=F
 ```
 
-- `-l` lines, `-w` words, `-c` bytes, `-m` characters (multibyte)
+- `-l` lines, `-w` words, `-c` bytes, `-m` characters (multibyte),
+  `-L` maximum line length (display width, tab stops every 8)
+- `--files0-from=F` NUL-terminated file lists from F (or stdin when F is `-`)
+- `--total=auto|always|only|never` control over the total row
+- `--debug` report which line-counting acceleration is in use
 - stdin, `-`, multiple files, `total` rows, GNU-compatible counts
+- A CLI surface modeled on GNU coreutils `wc` 9.11: same options, same
+  column alignment rule (counts are right-justified to the widest sum
+  of regular file sizes), same multibyte word semantics, same error
+  text. Swap it in and scripts keep working.
+- POSIXLY_CORRECT is honored: the no-break space stops being a word
+  separator, exactly like GNU wc.
 - no `--help` in fourteen languages. One `--help`, in English, the
   language of people who ship software