Remove shehu from TITLES: a Hausa given name, not a leading title

[derek73]

Closes #660.

shehu has been in TITLES since an early bulk import (1a51555f) and was never checked against decisions.md#vocabulary-collisions C-i. It fails C-i in the leading position, which is where the title claim acts:

Shehu Shagari   1.4.0 – 2.4.1: title 'Shehu', family 'Shagari'
                this PR:       given 'Shehu', family 'Shagari'

Changes

• nameparser/config/titles.py: removes shehu.
• docs/design/decisions.md: adds an Excluded (TITLES) entry with the counts from tools/name_usage.py shehu and the accepted cost (Shehu Abubakar, written as honorific plus name, reads given Shehu). The entry doesn't count leading-position use; it grounds the exclusion in CLDR's Hausa sample data, which files Shehu as the given name in all six renderings that contain it.
• docs/release_log.rst: adds a Fix bullet under Unreleased. If this ships as 2.4.2, move it into that entry.
• tools/name_usage.py: the docstring now says "a title until Shehu Shagari reads Shehu as a title: shehu is a common Hausa given name and an Albanian surname #660".

Measured

uv run python tools/differential/snapshot.py --since v2.4.1 lists 3 moved names, all held only by corpus_cldr.jsonl: Shehu Muhammed, Muhammed, Shehu and Modibbo, Shehu Bello. All three now agree with CLDR on every role and leave cldr_agreement.json. The JA locale rows were carried over unchanged (namedivider isn't installed locally). The 1.4.0 through 2.4.1 reading was checked on every released wheel.

No case row is added: it would pin a wordlist entry, not a fork, and the snapshot already holds the moved names.

Full suite passes without namedivider, matching CI's build jobs. mypy and ruff are clean. The design-docs review raised one point: the reason given for keeping shehu out of the graf class was asserted rather than measured. It is fixed in the second commit.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (affabb9) to head (66a9489).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #674   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          46       46           
  Lines        4534     4534           
=======================================
  Hits         4490     4490           
  Misses         44       44           

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