One select-row stitcher

[alecloudenback]

"Select rates followed by the ultimate tail, indexed by attained age" was implemented three times with three treatments of leading missing values: SelectMortality, the XTbML converter, and the CSV loader.

Changes

• New private _select_row(issue_age, select_rates, ultimate) next to SelectMortality. A select period longer than the ultimate table simply has no tail.
• SelectMortality maps _select_row over the rows.
• XTbML converter: per issue age, the rates vector preceded by duration - 1 leading missings, allocating a Union vector only when that count is nonzero, then _select_row.
• CSV extension: per row, trailing blanks are trimmed but a leading or interior blank is kept as missing, so a row with a gap is not silently truncated (the old backward scan did the same; a forward scan would not).

Baseline facts recorded before editing and now asserted: rows without leading missings stay Vector{Float64} (only rows with leading missings are Union{Missing,Float64}), cso2001.select[15][15] is missing, and omega(cso2001.select[98]) == 120 (issue #71).

Testing

• Unit tests for _select_row with leading missing, a select period ending exactly at omega, and one extending past omega.
• Unit test of the CSV row trimming with an interior blank.
• The SOA value-equality tests and the CSV-vs-XTbML equality tests pass unchanged.

---

Stacked on #143 (base branch refactor/08-vector-constructor). Merge the stack in order; once the base PR is merged and its branch deleted, GitHub retargets this PR to master.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JFnc6pvofuNXTW2xdCcewT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.93%. Comparing base (bcd9510) to head (31b9699).

Additional details and impacted files

@@                        Coverage Diff                         @@
##           refactor/08-vector-constructor     #144      +/-   ##
==================================================================
- Coverage                           91.05%   90.93%   -0.12%     
==================================================================
  Files                                  10       10              
  Lines                                 380      375       -5     
==================================================================
- Hits                                  346      341       -5     
  Misses                                 34       34              

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