Implement Singapore tax-benefit system

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GAPS.md

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+# PolicyEngine Singapore - Gaps & Adjustments Tracker
+
+Status: ~113 variables, ~155 parameter files, 122 tests (all passing)
+Branch: `implement-sg-tax-benefit-system` (PR #5)
+
+---
+
+## 1. Adjustments Needed to Existing Implementations
+
+### 1.1 ~~WIS Income-Based Tapering~~ RESOLVED
+Implemented exact piecewise linear formulas from CPF Board's
+official WIS calculator for all 4 age bands (30-34, 35-44,
+45-59, 60+). Each band has multi-segment ramp-up, plateau
+($1,700-$2,300), and ramp-down. 7 tests covering plateau,
+ramp-up, ramp-down, and boundary cases.
+
+### 1.2 ~~NSman Relief Categories~~ RESOLVED
+Added NsmanStatus enum (5 categories), is_wife_of_nsman and
+is_parent_of_nsman inputs. Formula uses select() over enum +
+max_() for mutual exclusivity (cannot combine self + wife/parent).
+Fixed key_appointment_non_active parameter: $3,000 -> $3,500.
+
+### 1.3 ~~Dependant Income Threshold Not Enforced~~ RESOLVED
+Added spouse_income input variable. spouse_relief.py now checks
+spouse_income <= dependant_income_threshold ($4k for 2024,
+$8k for 2025). Child/parent reliefs remain simplified (no
+individual dependant income tracking).
+
+### 1.4 ~~WMCR Uses Fixed Amounts Only~~ RESOLVED
+Added number_of_children_born_before_2024 input. Formula now
+splits: pre-2024 children use rate-based (15%/20%/25% of earned
+income), post-2024 children use fixed ($8k/$10k/$12k).
+
+### 1.5 ~~GSTV MediSave Income Ceiling~~ RESOLVED
+Verified: GSTV MediSave has NO assessable income ceiling (unlike
+GSTV Cash which has $39,000). The existing implementation is
+correct - it checks only citizenship, age >= 65, AV ceiling, and
+property count. Source: MOF press releases and GovBenefits.gov.sg.
+
+### 1.6 ~~PTR Parameters Unused~~ RESOLVED (BY DESIGN)
+Added clarifying comment to parenthood_tax_rebate.py explaining
+that per-child parameters (parenthood_first_child=$5k, etc.) exist
+as reference documentation only. PTR uses ptr_balance input because
+PolicyEngine cannot track multi-year carry-forward balances.
+
+### 1.7 ~~ComCare SMTA~~ RESOLVED (ELIGIBILITY ONLY)
+Added comcare_smta_eligible variable using the $800 monthly PCI
+benchmark. SMTA benefit amounts are determined by caseworker
+assessment (shortfall between income and basic living expenses),
+not a fixed formula, so only eligibility is modeled. The $800 PCI
+benchmark is officially described as "not a hard threshold" but
+is used as a proxy. Tests added to existing ComCare test file.
+
+### 1.8 ~~Silver Support CPF Savings Check~~ RESOLVED (DOCUMENTED)
+Added NOTE comment to silver_support_eligible.py documenting that
+real Silver Support also checks lifetime CPF savings. PolicyEngine
+cannot model CPF balances, so this check is omitted as a known
+limitation.
+
+---
+
+## 2. Missing Programs - High Priority
+
+### 2.1 ~~Baby Bonus Cash Gift~~ IMPLEMENTED
+$11,000 (1st-2nd child), $13,000 (3rd+). Requires married & citizen.
+4 tests.
+
+### 2.2 ~~CDA First Step Grant~~ IMPLEMENTED
+$5,000 (1st-2nd), $5,000/$10,000 (3rd+ pre/post Feb 2025).
+Requires citizen. 3 tests.
+
+### 2.3 ~~Childcare / Infant Care Subsidies~~ IMPLEMENTED
+Basic: $300/mo childcare, $600/mo infant care (working mothers).
+Additional: $467-$0/mo by GHI bracket. 4 tests.
+
+### 2.4 ~~KiFAS~~ IMPLEMENTED
+$163-$0/mo by GHI bracket. Requires citizen. 4 tests.
+
+### 2.5 ~~MOE Financial Assistance Scheme~~ IMPLEMENTED
+Eligibility variable: GHI <= $3,000 or PCI <= $750. 4 tests.
+
+### 2.6 ~~CHAS Tier~~ IMPLEMENTED
+Enum tiers: Blue (PCI <= $1,500), Orange (PCI <= $2,300),
+Green (all other citizens). 4 tests.
+
+### 2.7 MediShield Life Premium Subsidies (LOW)
+**Agency:** MOH / CPF
+**Subsidies:** Up to 50% premium subsidy (means-tested)
+**Modelability:** LOW - complex premium schedule by age + subsidy
+tiers by income. Skipped for now.
+
+### 2.8 ~~Enhanced CPF Housing Grant (EHG)~~ IMPLEMENTED
+Up to $120,000 for first-time buyers by GHI bracket. 4 tests.
+
+### 2.9 ~~FDW Levy Concession~~ IMPLEMENTED
+$300/mo standard, $60/mo concessionary. Concession if child < 16,
+elderly >= 67, or disabled member. 4 tests.
+
+### 2.10 ~~Seller's Stamp Duty (SSD)~~ IMPLEMENTED
+12%/8%/4% by holding period (1/2/3 years). 4 tests.
+
+---
+
+## 3. Missing Programs - Medium Priority
+
+### 3.1 ~~Assurance Package Cash~~ IMPLEMENTED
+$600 (low income <= $34k), $350 (mid <= $100k), $200 (higher).
+Multi-property owners get $200. 5 tests.
+
+### 3.2 Higher Education Bursaries (MOE/University) (DEFERRED)
+Varies by institution and income tier. Too many institution-specific
+schemes to model comprehensively.
+
+### 3.3 CareShield Life Premium Subsidies (DEFERRED)
+Requires income tier + premium schedule. Complex and data-limited.
+
+### 3.4 ~~Student Care Fee Assistance (SCFA)~~ IMPLEMENTED
+$290-$0/mo by GHI bracket. Requires citizen. 4 tests.
+
+### 3.5 SkillsFuture Credit (NOT MODELABLE)
+Balance tracking not possible in single-period architecture.
+
+### 3.6 ~~Edusave Contribution~~ IMPLEMENTED
+$230 (primary), $290 (secondary). Requires citizen + school level.
+4 tests.
+
+---
+
+## 4. Not Modelable (PolicyEngine Limitations)
+
+| Program | Reason |
+|---------|--------|
+| CPF LIFE payouts | Needs individual CPF balance history |
+| MediFund | Discretionary, case-by-case assessment |
+| ComCare SMTA amounts | Holistic assessment, no fixed formula |
+| WIS time-based tapering | Employment month tracking required |
+| PTR birth-order tracking | Multi-year carry-forward needed |
+| Progressive wage model | Sector-specific, employer-side |
+| SkillsFuture balance | Balance tracking across periods |
+| MediShield Life subsidies | Complex age+income premium matrix |
+| CareShield Life subsidies | Complex premium/subsidy schedule |
+
+---
+
+## 5. Implementation Summary
+
+### Currently Implemented (~105 variables)
+
+| Area | Variables | Tests |
+|------|-----------|-------|
+| Personal Income Tax | 20 (13 reliefs + rates + rebates + PTR) | 27 |
+| CPF Contributions | 3 (employee + employer + total) | 6 |
+| Property Tax | 1 (owner/non-owner occupied) | 3 |
+| Stamp Duty | 4 (BSD + ABSD + SSD + total) | 11 |
+| GST | 1 | 1 |
+| SDL | 1 | 2 |
+| GSTV | 4 (cash + U-Save + MediSave + S&CC) | 17 |
+| CDC Vouchers | 1 | 2 |
+| Assurance Package | 1 | 5 |
+| WIS | 3 (eligible + amount + cash) | 7 |
+| Silver Support | 2 (eligible + amount) | 3 |
+| ComCare | 3 (eligible + LTA + SMTA eligible) | 3 |
+| Baby Bonus | 1 (cash gift) | 4 |
+| CDA First Step | 1 | 3 |
+| Childcare Subsidy | 1 | 4 |
+| KiFAS | 1 | 4 |
+| MOE FAS | 1 (eligibility) | 4 |
+| Edusave | 1 | 4 |
+| CHAS | 1 (tier) | 4 |
+| EHG | 1 | 4 |
+| FDW Levy | 1 | 4 |
+| SCFA | 1 | 4 |
+| Input variables | ~59 | - |
+
+### Parameter Files: ~155
+### Total Tests: 122 (all passing)