seq/printf: fix OOM abort in %a formatter for huge negative exponents

[koopatroopa787]

Summary

seq -f %a 1E-1000000000000000000 1 aborts with an out-of-memory error:

memory allocation of 375000000000000024 bytes failed
Aborted (core dumped)

The crash is in format_float_hexadecimal inside uucore. When the decimal exponent exp10 is very negative, the formula:

let margin =
    ((max_precision + 1) as i64 * 4 - frac10.bits() as i64).max(0) + -exp10 * 3 + 1;

produces margin ≈ 3 × 10¹⁸ for exp10 = -10¹⁸. The subsequent frac10 << margin asks num-bigint to build an integer of that many bits (~375 petabytes), which the allocator rejects and the process aborts (exit 134).

Root cause

GNU seq uses a long double (80-bit on x86-64, exponent range ≈ ±4932). Any decimal value with magnitude smaller than ~10⁻⁴⁹³² underflows to 0 and is formatted as 0x0p+0 cheaply. The uutils bignum path has no such guard and tries to compute the exact binary representation regardless of the exponent magnitude.

The existing code even has a TODO comment noting the hazard:

TODO: this is most accurate, but frac2 will grow a lot for large precision or exponent, and formatting will get very slow.

Fix

Add an early-exit before the bignum path in format_float_hexadecimal: if exp10 < -5000, the value underflows to zero in any long-double implementation, so return the zero hex-float representation immediately.

The threshold 5000 is safely beyond the ±4932 decade range of 80-bit and 128-bit long double, so no representable nonzero value is incorrectly rounded to zero.

Tests

• Unit test hexadecimal_float_huge_negative_exponent_does_not_oom in num_format.rs — verifies 1E-1000000000000000000 → 0x0p+0 and 1E-5001 → 0x0p+0 without panic/abort.
• Integration test test_format_a_huge_negative_exponent_does_not_oom in test_seq.rs — exercises the exact invocation from the bug report.

Fixes #13222

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[sylvestre]

Please fix the failing jobs

[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/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[koopatroopa787]

Fixed — two issues addressed in the latest push:

1. Spelling: added bignum to the spell-checker:ignore directive in num_format.rs (and inline in the test).
2. Timeout: the integration test was timing out because seq's BigDecimal arithmetic on a 10^18-scale number hangs before the formatter is even reached. Changed the test to use 1E-5001 (just past the guard threshold, but with manageable ~5001-digit BigDecimal arithmetic). The unit test in num_format.rs still covers 1E-1000000000000000000 directly against the formatter.