feat: add logic functions (default, coalesce, ternary, in_range)

Implement 4 conditional logic functions for enhanced template control:
- default(value, default) - Return default if value is falsy
- coalesce(values) - Return first non-null value
- ternary(condition, true_val, false_val) - Ternary operator
- in_range(value, min, max) - Check if value in range (inclusive)

The default function treats the following as falsy: null, undefined,
false, 0, empty string, and empty arrays. The ternary function uses
MiniJinja's is_true() for consistent truthiness evaluation.

Features:
- Comprehensive falsy value detection in default()
- Array-based value precedence in coalesce()
- Truthy/falsy evaluation in ternary()
- Inclusive range checking with floats support
- Error handling for invalid inputs

Testing:
- 36 unit tests in tests/test_logic_functions.rs
- 37 integration test cases in tests/integration/tests/20_logic_functions.sh
- Combined use cases demonstrating real-world patterns
- All tests passing, clippy clean

Use cases:
- Configuration fallbacks and defaults
- Multi-level precedence (env -> config -> default)
- Conditional rendering based on dynamic values
- Resource usage validation and range checking
- Environment-based configuration switching

Files created:
- src/functions/logic.rs - Logic function implementations
- tests/test_logic_functions.rs - Unit tests
- tests/integration/tests/20_logic_functions.sh - Integration tests

Updated:
- README.md - Added Logic Functions section with documentation
- TODO.md - Marked 4 logic functions as complete
- src/functions/mod.rs - Registered logic functions

All tests passing, clippy clean.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: bordeux

README.md

@@ -2570,6 +2570,133 @@ Python files:
 {% endif %}
 ```
 
+### Logic Functions
+
+Conditional logic and default value handling.
+
+#### `default(value, default)`
+
+Return default value if the provided value is falsy.
+
+**Arguments:**
+- `value` (required): Value to check
+- `default` (required): Default value to return if value is falsy
+
+**Returns:** The value if truthy, otherwise the default
+
+**Falsy values:** `null`, `undefined`, `false`, `0`, empty string `""`, empty array `[]`
+
+**Example:**
+```jinja
+{# Use default for empty string #}
+{{ default(value="", default="N/A") }}
+{# Output: N/A #}
+
+{# Use actual value if truthy #}
+{{ default(value="Hello", default="N/A") }}
+{# Output: Hello #}
+
+{# Configuration with defaults #}
+{% set config = {"port": 8080} %}
+Host: {{ default(value=config.host, default="localhost") }}
+Port: {{ default(value=config.port, default=3000) }}
+```
+
+#### `coalesce(values)`
+
+Return the first non-null value from an array.
+
+**Arguments:**
+- `values` (required): Array of values to check
+
+**Returns:** First value that is not null/undefined, or null if all are null
+
+**Example:**
+```jinja
+{# Find first non-null value #}
+{% set a = none %}
+{% set b = none %}
+{% set c = "found" %}
+{{ coalesce(values=[a, b, c]) }}
+{# Output: found #}
+
+{# Configuration precedence #}
+{% set env_host = none %}
+{% set config_host = "prod.example.com" %}
+{% set default_host = "localhost" %}
+Host: {{ coalesce(values=[env_host, config_host, default_host]) }}
+{# Output: Host: prod.example.com #}
+```
+
+#### `ternary(condition, true_val, false_val)`
+
+Ternary operator - return one value based on a condition.
+
+**Arguments:**
+- `condition` (required): Boolean condition to evaluate
+- `true_val` (required): Value to return if condition is true
+- `false_val` (required): Value to return if condition is false
+
+**Returns:** `true_val` if condition is truthy, otherwise `false_val`
+
+**Example:**
+```jinja
+{# Simple ternary #}
+{{ ternary(condition=true, true_val="Yes", false_val="No") }}
+{# Output: Yes #}
+
+{# With comparison #}
+{% set score = 85 %}
+Result: {{ ternary(condition=score >= 60, true_val="Pass", false_val="Fail") }}
+{# Output: Result: Pass #}
+
+{# Status indicator #}
+{% set cpu_usage = 75 %}
+Status: {{ ternary(
+  condition=cpu_usage > 90,
+  true_val="Critical",
+  false_val="Normal"
+) }}
+```
+
+#### `in_range(value, min, max)`
+
+Check if a numeric value is within a range (inclusive).
+
+**Arguments:**
+- `value` (required): Numeric value to check
+- `min` (required): Minimum value (inclusive)
+- `max` (required): Maximum value (inclusive)
+
+**Returns:** `true` if min <= value <= max, `false` otherwise
+
+**Example:**
+```jinja
+{# Check if in range #}
+{{ in_range(value=50, min=0, max=100) }}
+{# Output: true #}
+
+{# Validate port number #}
+{% set port = 8080 %}
+{% if in_range(value=port, min=1024, max=65535) %}
+  Valid port number
+{% else %}
+  Invalid port number
+{% endif %}
+
+{# Temperature range check #}
+{% set temp = 22 %}
+Comfortable: {{ in_range(value=temp, min=18, max=25) }}
+
+{# Resource usage validation #}
+{% set cpu = 75.5 %}
+{% if in_range(value=cpu, min=0, max=80) %}
+  CPU usage normal
+{% else %}
+  CPU usage high
+{% endif %}
+```
+
 ### Math Functions
 
 Perform mathematical calculations and operations.

TODO.md

@@ -210,10 +210,10 @@ This document contains ideas for new functions and features to make tmpltool mor
 *Enhanced conditional logic*
 
 **General Logic:**
-- [ ] `default(value, default)` - Return default if value is falsy
-- [ ] `coalesce(values...)` - Return first non-null value
-- [ ] `ternary(condition, true_val, false_val)` - Ternary operator
-- [ ] `in_range(value, min, max)` - Check if value in range
+- [x] `default(value, default)` - Return default if value is falsy
+- [x] `coalesce(values...)` - Return first non-null value
+- [x] `ternary(condition, true_val, false_val)` - Ternary operator
+- [x] `in_range(value, min, max)` - Check if value in range
 
 **Array Predicates:**
 - [x] `array_any(array, predicate)` - Check if any item matches