od: say the skip ran past the end of the combined input

[Socialpranker]

-j/--skip-bytes is measured over all the inputs joined together, so the
message naming a single "input" is misleading once more than one file is
given:

$ printf abc > a; printf de > b

$ od -j6 a b          # GNU
od: cannot skip past end of combined input
$ od -j6 a b          # uutils, before
od: tried to skip past end of input

GNU prints that one wording for every shape of the failure — one file,
several files, stdin, with or without -N — so this is a straight string
change in src/uu/od/locales/en-US.ftl, with the French translation
updated to match.

How the GNU behavior was established

By running the installed GNU coreutils 9.11 binary (Homebrew, god) as a
black box: -j past the end of one file, of two files, of piped stdin,
combined with -N, and the boundary case -j5 a b that must still
succeed. Exit status and stderr diffed against uutils. I did not read GNU
coreutils source.

Testing

• New test_skip_bytes_past_end_message in tests/by-util/test_od.rs
  pins the exact stderr and exit status 1 for the two-file case.
  Mutation-checked: reverting the .ftl line alone makes it fail.
• cargo test --features od --test tests test_od: 78 passed, 0 failed.
• cargo clippy -p uu_od --all-targets -- -D warnings: clean.
• cargo fmt --check: clean.
• Differential A/B against GNU coreutils 9.11 over the 72 od invocations
  in my harness: mismatches 2 -> 0.

Not in scope

Two neighbouring differences left alone: od -j 1x f reports
invalid -j argument '1x' where GNU reports
invalid suffix in -j argument '1x', and when a named file is missing
uutils prints this skip error after the No such file or directory line
while GNU stops at the first.

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:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/pipe-f is now passing!