feat: add String.fromByte primitive with validation and unit tests

Implement a new String.fromByte(_) static method that creates a single-byte string from an integer 0-255. Includes runtime validation for non-integer, non-numeric, negative, and out-of-range values. Adds the underlying wrenStringFromByte helper in wren_value.c and exposes it via a new DEF_PRIMITIVE in wren_core.c. Documents the method in the core string markdown and provides six test cases covering happy path and all error conditions.

test/core/string/from_byte.wren

@@ -0,0 +1,3 @@
+System.print(String.fromByte(65))   // expect: A
+System.print(String.fromByte(0).bytes[0])   // expect: 0
+System.print(String.fromByte(255).bytes[0])   // expect: 255

test/core/string/from_byte_not_int.wren

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+System.print(String.fromByte(12.34))   // expect runtime error: Byte must be an integer.

test/core/string/from_byte_not_num.wren

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+System.print(String.fromByte("not num"))   // expect runtime error: Byte must be a number.

test/core/string/from_byte_too_large.wren

@@ -0,0 +1 @@
+System.print(String.fromByte(0xff + 1))   // expect runtime error: Byte cannot be greater than 0xff.

test/core/string/from_byte_too_small.wren

@@ -0,0 +1 @@
+System.print(String.fromByte(-1))   // expect runtime error: Byte cannot be negative.