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/template.toml

@@ -0,0 +1,25 @@
+[template]
+name = "weather"
+base = "basic"
+description = "Weather station with TMP36 temperature sensor"
+
+[requires]
+libraries = ["tmp36"]
+board_capabilities = ["analog"]
+
+# Default pin assignments per board.
+# "default" is used when no board-specific section exists.
+[pins.default]
+tmp36_data = { pin = "A0", mode = "analog" }
+
+[pins.uno]
+tmp36_data = { pin = "A0", mode = "analog" }
+
+[pins.mega]
+tmp36_data = { pin = "A0", mode = "analog" }
+
+[pins.nano]
+tmp36_data = { pin = "A0", mode = "analog" }
+
+[pins.leonardo]
+tmp36_data = { pin = "A0", mode = "analog" }