feat: implement nested functions, collections.namedtuple, and tuple star-unpack (#507, #508, #509)

Phase 1 — Tuple star-unpack (#509):
- Fix GenerateStarUnpacking to handle TupleType via ItemN access
- Fix CheckStarUnpacking rest element type for heterogeneous tuples
- Add tuple_star_unpack_tail and tuple_star_unpack_middle test fixtures

Phase 2 — Nested functions (#507):
- Register nested FunctionSymbols in enclosing scope during type checking
- Emit nested functions as C# local functions (LocalFunctionStatementSyntax)
- Save/restore method scope state around local function generation
- Add nested_function_basic and nested_function_closure test fixtures

Phase 3 — collections.namedtuple (#508):
- Add Namedtuple stub to Sharpy.Core Collections module
- Detect namedtuple("X", [...]) pattern in TypeChecker, synthesize TypeSymbol
- Generate C# record class with ToString() override for Python repr format
- Exempt namedtuple assignments from module-level statement validation
- Add namedtuple_basic, namedtuple_print, match_named_tuple test fixtures

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antonsynd

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/namedtuple_basic.expected

@@ -0,0 +1,2 @@
+1
+2

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/namedtuple_basic.spy

@@ -0,0 +1,8 @@
+from collections import namedtuple
+
+Point = namedtuple("Point", ["x", "y"])
+
+def main():
+    p = Point(1, 2)
+    print(p.x)
+    print(p.y)

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/namedtuple_print.expected

@@ -0,0 +1 @@
+Color(r=255, g=128, b=0)

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/namedtuple_print.spy

@@ -0,0 +1,7 @@
+from collections import namedtuple
+
+Color = namedtuple("Color", ["r", "g", "b"])
+
+def main():
+    c = Color(255, 128, 0)
+    print(c)

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/tuple_star_unpack_middle.expected

@@ -0,0 +1,3 @@
+1
+[2, 3, 4]
+5

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/tuple_star_unpack_middle.spy

@@ -0,0 +1,5 @@
+def main():
+    first, *mid, last = (1, 2, 3, 4, 5)
+    print(first)
+    print(mid)
+    print(last)

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/tuple_star_unpack_tail.expected

@@ -0,0 +1,2 @@
+[1, 2, 3]
+4

src/Sharpy.Compiler.Tests/Integration/TestFixtures/expressions/tuple_star_unpack_tail.spy

@@ -0,0 +1,4 @@
+def main():
+    *rest, last = (1, 2, 3, 4)
+    print(rest)
+    print(last)

src/Sharpy.Compiler.Tests/Integration/TestFixtures/functions/nested_function_basic.expected

@@ -0,0 +1,2 @@
+Hello, World!
+Hello, Sharpy!

src/Sharpy.Compiler.Tests/Integration/TestFixtures/functions/nested_function_basic.spy

@@ -0,0 +1,6 @@
+def main():
+    def greet(name: str) -> str:
+        return f"Hello, {name}!"
+
+    print(greet("World"))
+    print(greet("Sharpy"))