shuf: report a bad -i range the way GNU does

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A LO-HI that would not parse was rejected by clap's value parser, which
names the option and the internal reason:

$ shuf -i 5-1
shuf: invalid input range: '5-1'                                             # GNU
error: invalid value '5-1' for '--input-range <LO-HI>': start exceeds end    # uutils, before

$ shuf -i 1-
shuf: invalid input range: '1-'                                              # GNU
error: invalid value '1-' for '--input-range <LO-HI>': cannot parse integer  # uutils, before
from empty string

$ shuf -i 99999999999999999999999-1
shuf: invalid input range: '99999999999999999999999-1': Value too large to be stored in data type

GNU names the range as a whole whatever is wrong with it — a missing -,
a non-numeric bound, an extra -, LO above HI — and adds a detail in
exactly one case, a bound that does not fit.

The fix

parse_range moves out of .value_parser() and into uumain, returning
a UResult with USimpleError::new(1, ...). IntErrorKind::PosOverflow
selects the "Value too large" wording; everything else gets the plain one.
The two message keys replace shuf-error-start-exceeds-end and
shuf-error-missing-dash, which had no other user.

Five existing assertions pinned the clap wording; they are updated.

How the GNU behavior was established

By running the installed GNU coreutils 9.11 binary (Homebrew, gshuf) as
a black box over eight -i values — 5-1, abc, 1-, 1-2-3, -3,
the empty string, an overflowing bound, and the valid 3-3 — diffing
stderr and exit status against uutils. Seven of the eight now match. I did
not read GNU coreutils source.

Testing

• New test_shuf_invalid_input_range_message in
  tests/by-util/test_shuf.rs: five malformed ranges with the exact
  stderr and exit status 1, plus the overflow case. Mutation-checked:
  reverting shuf.rs and the .ftl makes six tests fail.
• cargo test --features shuf --test tests test_shuf: 75 passed, 0 failed.
• cargo clippy -p uu_shuf --all-targets -- -D warnings: clean.
• cargo fmt --check: clean.
• Differential A/B against GNU coreutils 9.11 over the 15 shuf
  invocations in my harness: mismatches 1 -> 0.

Not in scope

shuf -i -3 still stops at clap with unexpected argument '-3' found
where GNU takes -3 as the value of -i. That is about hyphen handling
in the parser, not about the message, and is left alone.

Disclosure

Prepared with AI assistance (Claude Opus 5, via Claude Code), per the AI
policy in CONTRIBUTING.md. Every GNU behavior quoted above came from
running the installed binary, not from reading GPL source. All testing was
run locally.

[github-actions]

GNU testsuite comparison:

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Congrats! The gnu test tests/sort/sort-merge-fdlimit is no longer failing!
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/rm/many-dir-entries-vs-OOM is now passing!
Note: The gnu test tests/dd/fail-ftruncate-fstat was skipped on 'main' but is now failing.