Fix intword() rounding carry for very large numbers

[binggao1230]

Problem

intword() decides whether rounding pushed a value up into the next magnitude
with:

if not largest_ordinal and rounded_value * power == powers[ordinal + 1]:

For values above ~10**22, rounded_value * power is evaluated in floating
point and no longer equals the exact powers[ordinal + 1], so the carry is
skipped and the number is rendered against the lower magnitude:

>>> import humanize
>>> humanize.intword(10**24 - 1)
'1000.0 sextillion'      # expected '1.0 septillion'
>>> humanize.intword(10**27 - 1)
'1000.0 septillion'      # expected '1.0 octillion'

The same happens at 10**30 and 10**33.

Fix

Compare rounded_value against the exact integer ratio
powers[ordinal + 1] // power, keeping the check in exact integer terms rather
than relying on a large float product. This mirrors the recently fixed carry
handling in metric() (#328) and naturalsize() (#329).

Tests

test_intword_rounding_rollover asserts the correct roll-over across every
affected magnitude (10**24/27/30/33 - 1), plus a couple of just-below cases
that must not roll over, guarding both directions.

pytest → 716 passed. ruff check / ruff format --check clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (4c85c35) to head (eda405b).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #346   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files          12       12           
  Lines         913      925   +12     
=======================================
+ Hits          909      921   +12     
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.62% <100.00%> (+0.03%)	⬆️
ubuntu-latest	97.62% <100.00%> (+0.03%)	⬆️
windows-latest	95.78% <100.00%> (+0.05%)	⬆️

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[codspeed]

Merging this PR will not alter performance

✅ 15 untouched benchmarks

---

_{Comparing gaoflow:fix-intword-carry-rollover (eda405b) with main (4c85c35)}

[hugovk]

import humanize

print(humanize.intword(10**24 - 1))
print(humanize.intword(10**33 - 1))
print(humanize.intword(-(10**24 - 1)))
print(humanize.intword(10**24 - 1, "%.0f"))

Before

1000.0 sextillion
1000.0 nonillion
-1000.0 sextillion
1000 sextillion

After

1.0 septillion
1.0 decillion
-1.0 septillion
1 septillion