Layer 3: Templates as pure data, weather template, .anvilignore refresh system

Templates are now composed declaratively via template.toml -- no Rust code
changes needed to add new templates. The weather station is the first
composed template, demonstrating the full pattern.

Template engine:
- Composed templates declare base, required libraries, and per-board pins
- Overlay mechanism replaces base files (app, sketch, tests) cleanly
- Generic orchestration: extract base, apply overlay, install libs, assign pins
- Template name tracked in .anvil.toml for refresh awareness

Weather template (--template weather):
- WeatherApp with 2-second polling, C/F conversion, serial output
- TMP36 driver: TempSensor interface, Tmp36 impl, Tmp36Mock, Tmp36Sim
- Managed example tests in test_weather.cpp (unit + system)
- Minimal student starters in test_unit.cpp and test_system.cpp
- Per-board pin defaults (A0 for uno, A0 for mega, A0 for nano)

.anvilignore system:
- Glob pattern matching (*, ?) with comments and backslash normalization
- Default patterns protect student tests, app code, sketch, config
- anvil refresh --force respects .anvilignore
- anvil refresh --force --file <path> overrides ignore for one file
- anvil refresh --ignore/--unignore manages patterns from CLI
- Missing managed files always recreated even without --force
- .anvilignore itself is in NEVER_REFRESH (cannot be overwritten)

Refresh rewrite:
- Discovers all template-produced files dynamically (no hardcoded list)
- Extracts fresh template + libraries into temp dir for byte comparison
- Config template field drives which files are managed
- Separated missing-file creation from changed-file updates

428 tests passing on Windows MSVC, 0 warnings.

templates/weather/__name__/__name__.ino.tmpl

@@ -0,0 +1,35 @@
+/*
+ * {{PROJECT_NAME}}.ino -- TMP36 weather station
+ *
+ * Reads temperature from a TMP36 sensor every 2 seconds
+ * and prints to Serial:
+ *
+ *     Temperature: 23.5 C (74.3 F)
+ *
+ * All logic lives in lib/app/{{PROJECT_NAME}}_app.h which depends
+ * on the HAL and TempSensor interfaces, making it fully testable
+ * on the host without hardware.
+ *
+ * Wiring (TMP36, flat side facing you):
+ *   Pin 1 (left)   -> 5V
+ *   Pin 2 (middle) -> A0 (analog input)
+ *   Pin 3 (right)  -> GND
+ *
+ * Serial: 115200 baud
+ */
+
+#include <hal_arduino.h>
+#include <{{PROJECT_NAME}}_app.h>
+#include <tmp36_analog.h>
+
+static ArduinoHal hw;
+static Tmp36Analog sensor(&hw, A0);
+static WeatherApp app(&hw, &sensor);
+
+void setup() {
+    app.begin();
+}
+
+void loop() {
+    app.update();
+}