ENH: Standardize cell naming in TradLife_A.BaseProj

[fumitoh]

Summary

Standardize the cell naming convention in TradLife_A's BaseProj (and the four mirror cells in PV) so naming reflects the role each cell plays:

• claims is now the umbrella term for benefit cashflows (was benefit_total).
• Individual benefit components use claims_* (was benefit_*).
• Per-policy values use a _pp suffix (was size_*), with size_benefit_* → claims_*_pp as the combined case.
• Existing per-policy cells without size_ get the _pp suffix too: claim_pp → claims_death_pp, expense_acq → expense_acq_pp, expense_maint → expense_maint_pp.
• The PV mirror cells are renamed for parity: pv_claims → pv_claims_death, pv_benefit_total → pv_claims, pv_benefit_mat → pv_claims_mat, pv_benefit_surr → pv_claims_surr.

Files touched

• lifelib/libraries/annuallife/TradLife_A/BaseProj/__init__.py — cell definitions, internal references, module docstring.
• lifelib/libraries/annuallife/TradLife_A/PV/__init__.py — mirror-cell renames + body updates referencing renamed BaseProj cells.
• lifelib/libraries/annuallife/plot_tradlife_a.py — vars list strings.
• lifelib/libraries/annuallife/plot_pvcashflows.py — vars list strings.
• lifelib/libraries/annuallife/tradlife_a-demo.ipynb — same vars list strings.
• makedocs/source/libraries/annuallife/BaseProj.rst — autosummary/autofunction listings.
• makedocs/source/libraries/annuallife/PV.rst — autosummary/autofunction listings.

Test plan

• pytest lifelib/tests/libraries/test_tradlife_a.py — test_pv_net_cf_matches_simplelife passes for all three policy pairs.
• Smoke check: all 37 new cell names load and produce sensible numerical values; all 13 old cell names (benefit_total, benefit_*, size_*, size_benefit_*, claim_pp, expense_acq, expense_maint, pv_benefit_*) are gone from the loaded model.
• String-keyed cell access used by both plot scripts and the demo notebook (proj.cells['claims_surr'](t), proj.cells['claims_death'](t), proj.cells['pv_claims_surr'](t), proj.cells['pv_claims_death'](t)) resolves and yields cashflows.

Notes for reviewers

• Module docstrings in BaseProj/__init__.py were updated so the prose still matches the convention (e.g. the old Size and Benefit sections are now _pp and Claims).
• An unrelated pre-existing issue surfaced during smoke checks: invst_income_pp (formerly size_invst_income) chains into reserve_prem_rsrv_aft_mat_pp, which calls pol.ReserveNLP_Rate('VAL', t) — that attribute is missing on the policy interface. The formula body is unchanged from the pre-rename code, and test_pv_net_cf_matches_simplelife doesn't traverse this path. Out of scope for this PR.

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