feat: Implement full problem generation

This commit marks a major milestone in the problem generator project.

Key systems include:
- Fully functional problem generation for all 23 defined concepts (Simple Interest, Compound Interest, Banker's Discount, Effective Rate, Continuous Compounding, Exact/Ordinary Simple Interest).
- Robust date handling for date-specific interest calculations.
- Improved solution presentation with accurate "Substitute Values" step and complete variable descriptions.
- Interactive problem display in `main.py`: problem statement shown first, question and solution revealed on user input.
- Added plausibility checks for rate calculations to avoid unrealistic negative rates.
- Comprehensive `README.md` update:
    - Detailed system architecture (modules, data files, Mermaid diagram).
    - List of all currently covered financial concepts.
    - Instructions for running and extending the generator.
    - Discussion of scope for future enhancements (Equation of Value, Gradients, etc.).
- Refinements to `value_sampler.py` for better formatting and handling of None values.
- Updates to `text_snippets.json` for complete variable descriptions and improved solution step phrasing.
- Updates to `value_ranges.json` for date generation parameters.
- `problem_engine.py` now systematically tests all concepts when run directly.
- Added `uv.lock` to track resolved dependencies.

The system is capable of generating a wide variety of engineering economy problems with detailed, step-by-step solutions and an interactive user experience.

data/value_ranges.json

@@ -55,6 +55,13 @@
     "internal_precision": 8,
     "display_precision": 2
   },
+  "interest_rate_per_period_effective": {
+    "min": 0.005,
+    "max": 0.05,
+    "unit_display": "% per period",
+    "internal_precision": 8,
+    "display_precision": 3
+  },
   "discount_rate_bankers": {
     "min": 0.05,
     "max": 0.20,
@@ -83,7 +90,7 @@
   },
   "number_of_periods_general": {
     "min": 2,
-    "max": 30,
+    "max": 60,
     "unit": "periods",
     "integer": true
   },
@@ -111,20 +118,24 @@
     "description": "General date, used for start_date and end_date generation.",
     "type": "date",
     "min_year": 1990,
-    "max_year": 2030 
+    "max_year": 2030
   },
   "time_days_exact_ordinary": {
     "description": "Number of days for exact/ordinary interest calculations (derived from dates).",
     "type": "integer",
-    "min": 30, 
-    "max": 730, 
+    "min": 30,
+    "max": 730,
     "unit": "days",
     "display_precision": 0
   },
   "time_base_days_exact_ordinary": {
     "description": "Time base in days for exact (365/366) or ordinary (360) interest.",
     "type": "integer",
-    "options": [360, 365, 366], 
+    "options": [
+      360,
+      365,
+      366
+    ],
     "unit": "days",
     "display_precision": 0
   },
@@ -132,6 +143,13 @@
     "description": "Time period as a fraction of a year (calculated).",
     "type": "float",
     "unit": "years",
-    "display_precision": 6 
+    "display_precision": 6
+  },
+  "date_period_generation": {
+    "description": "Parameters for generating random date periods.",
+    "min_days": 30,
+    "max_days": 730,
+    "base_year_start": 1990,
+    "base_year_end": 2030
   }
 }