fix(printf): compute asterisk width magnitude in unsigned arithmetic

[bolverk]

Fixes #13766

Summary

A negative * field-width argument of i64::MIN (printf '%*d' -9223372036854775808 1) made resolve_asterisk_width panic with attempt to negate with overflow: the old code computed the magnitude as -(nb as isize), and |i64::MIN| = 2^63 is not representable in i64/isize.

The fix computes the magnitude with nb.unsigned_abs(), which is designed for exactly this case and returns 2^63 in unsigned (u64) arithmetic.

Behavior

• Before: panic / exit 134 (attempt to negate with overflow)
• After: the width 2^63 exceeds MAX_FORMAT_WIDTH, so printf fails gracefully with printf: write error: formatting width too large (exit 1), matching the handling of other oversized widths.

Verification

Formal verification of the width-resolution arithmetic was done in Dafny (4.11): it specifies the intended behavior (negative width ⇒ left-align with magnitude), proves |i64::MIN| overflows i64 but fits u64/usize, and proves the unsigned_abs()-based implementation is overflow-free and matches the spec. Proof: 15 verified, 0 errors.

Tests

• New unit test resolve_asterisk_width::asterisk_i64_min_width (both SignedInt and Unparsed argument forms)
• New integration test test_printf::test_asterisk_width_i64_min_no_panic
• Full printf suite passes: 131 passed, 0 failed, 1 ignored; cargo fmt and clippy clean.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[bolverk]

Updated the integration test to be portable: on 32-bit targets (usize = 32 bits) |i64::MIN| = 2^63 does not fit, so the width is clamped to 0 (no padding) and printf succeeds with |1|; on 64-bit targets it exceeds MAX_FORMAT_WIDTH and fails with a write error. In both cases the original panic ("attempt to negate with overflow") is gone. Verified locally on both x86_64-musl and i686-musl (131/131 printf tests pass on each).

[sylvestre]

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