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/lib/app/__name___app.h.tmpl

@@ -0,0 +1,100 @@
+#ifndef APP_H
+#define APP_H
+
+#include <hal.h>
+#include "tmp36.h"
+
+#include <stdio.h>
+
+/*
+ * WeatherApp -- Reads a TMP36 temperature sensor and reports to Serial.
+ *
+ * Every READ_INTERVAL_MS (2 seconds), reads the temperature and prints
+ * a formatted line like:
+ *
+ *     Temperature: 23.5 C (74.3 F)
+ *
+ * The sensor is injected through the TempSensor interface, so this
+ * class works with real hardware, mocks, or simulations.
+ *
+ * Wiring (TMP36):
+ *   Pin 1 (left, flat side facing you)  -> 5V
+ *   Pin 2 (middle)                      -> A0 (analog input)
+ *   Pin 3 (right)                       -> GND
+ */
+class WeatherApp {
+public:
+    static constexpr unsigned long READ_INTERVAL_MS = 2000;
+
+    WeatherApp(Hal* hal, TempSensor* sensor)
+        : hal_(hal)
+        , sensor_(sensor)
+        , last_read_ms_(0)
+        , last_celsius_(0.0f)
+        , last_fahrenheit_(0.0f)
+        , read_count_(0)
+    {}
+
+    // Call once from setup()
+    void begin() {
+        hal_->serialBegin(115200);
+        hal_->serialPrintln("WeatherApp started");
+        last_read_ms_ = hal_->millis();
+        readAndReport();
+    }
+
+    // Call repeatedly from loop()
+    void update() {
+        unsigned long now = hal_->millis();
+        if (now - last_read_ms_ >= READ_INTERVAL_MS) {
+            readAndReport();
+            last_read_ms_ = now;
+        }
+    }
+
+    // -- Accessors for testing ------------------------------------------------
+    float lastCelsius()    const { return last_celsius_; }
+    float lastFahrenheit() const { return last_fahrenheit_; }
+    int   readCount()      const { return read_count_; }
+
+private:
+    void readAndReport() {
+        last_celsius_    = sensor_->readCelsius();
+        last_fahrenheit_ = sensor_->readFahrenheit();
+        read_count_++;
+
+        // Format: "Temperature: 23.5 C (74.3 F)"
+        // Use integer math for AVR compatibility (no %f on AVR printf)
+        char buf[48];
+        formatOneDp(buf, sizeof(buf), last_celsius_);
+        hal_->serialPrint("Temperature: ");
+        hal_->serialPrint(buf);
+        hal_->serialPrint(" C (");
+        formatOneDp(buf, sizeof(buf), last_fahrenheit_);
+        hal_->serialPrint(buf);
+        hal_->serialPrintln(" F)");
+    }
+
+    // Format a float as "23.5" or "-5.3" using integer math only.
+    static void formatOneDp(char* buf, size_t len, float value) {
+        int whole = (int)value;
+        int frac  = (int)(value * 10.0f) % 10;
+        if (frac < 0) frac = -frac;
+
+        // Handle -0.x case (e.g. -0.3 C)
+        if (value < 0.0f && whole == 0) {
+            snprintf(buf, len, "-0.%d", frac);
+        } else {
+            snprintf(buf, len, "%d.%d", whole, frac);
+        }
+    }
+
+    Hal*        hal_;
+    TempSensor* sensor_;
+    unsigned long last_read_ms_;
+    float       last_celsius_;
+    float       last_fahrenheit_;
+    int         read_count_;
+};
+
+#endif // APP_H