Preserve trailing ANSI reset codes in ansi_trim_end

[MsfPablo]

Background

While looking into #769 (extraneous trailing spaces with --color=always) I found that the reported symptom is already fixed on master by bb7d47c (Fix double-spaced TTY output caused by ANSI-unaware trim_end, for #848) — colored output no longer has trailing spaces. #769 can probably be closed.

That fix, however, left a related defect behind.

Problem

ansi_trim_end() computes the visible width of the trimmed row and then calls truncate() with that width. truncate() stops as soon as the width budget is exhausted, so any escape sequence that sits after the trimmed padding is discarded along with the spaces.

Rows are assembled by wrapping each padded column in apply_color(), which puts the reset code at the very end of the styled span. Trimming therefore removes the reset that closed the last column, leaving an unterminated SGR sequence on every line — the final row's color bleeds into whatever is printed next (typically the shell prompt).

On master:

$ procs --color=always zsh | tail -1 | cat -v | tail -c 20
/bin/zsh -l          # <- no ESC[0m; the terminal stays colored afterwards

Reduced:

ansi_trim_end("\x1b[31mabc   \x1b[0m")
// master:   "\x1b[31mabc"        <- reset lost
// expected: "\x1b[31mabc\x1b[0m"

Fix

Trim the trailing whitespace directly rather than routing through truncate(), keeping every escape sequence that follows the trimmed run. Behavior for the whitespace itself is unchanged, so the #848/#769 fix is preserved.

Tests

Adds src/util.rs unit tests for ansi_trim_end: plain strings, padding inside a styled span, reset-code preservation, multiple styled columns, whitespace-only input, and wide/box-drawing characters.

5 of the 6 fail on master and pass with this change:

test util::tests::test_ansi_trim_end_keeps_reset_code ... FAILED
test util::tests::test_ansi_trim_end_removes_padding_inside_style ... FAILED
test util::tests::test_ansi_trim_end_multiple_columns ... FAILED
test util::tests::test_ansi_trim_end_only_whitespace ... FAILED
test util::tests::test_ansi_trim_end_wide_chars ... FAILED

Verification

$ cargo test
test result: ok. 18 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

$ cargo fmt --check
(clean)

$ cargo clippy --all-targets -- -D warnings
14 findings, byte-for-byte the same set as the git-stash baseline
(only line numbers in util.rs shift). No new lint introduced.

End-to-end assertion over procs --color=always output — 12 lines, 0 with trailing whitespace, 0 with an unterminated SGR sequence.

Tested on macOS (aarch64).

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Disclosure: this patch was written with AI assistance (Claude). I reviewed the diff, ran the test/fmt/clippy commands above myself, and isolated the pre-existing clippy findings against a clean baseline.