fix(lint): replace 3.14 test literals with 2.5 to clear clippy::approx_constant (refs #22) (#23)

`clippy::approx_constant` is denied-by-default; the five `3.14` literals
in lib/common test asserts (math.rs:268, string.rs:300, types.rs:297,
types.rs:304) were tripping it and causing `cargo clippy -p my-lang
--lib --tests --no-deps` to exit non-zero, which is a CI gate hazard
for any future `-D warnings` rollout.

None of the affected sites were testing pi specifically -- the literal
was incidental. Swapping to `2.5` keeps the tests round-trip-clean
(parses to itself, formats to itself, exact in IEEE-754) without
approximating any well-known constant.

Verified with `cargo test -p my-lang --lib library::common::` (39/39
pass, including the four affected tests). The wider lib test suite is
not runnable on clean main yet -- it crashes with STATUS_STACK_BUFFER_OVERRUN
on the depth-overflow tests fixed in #21, which is why this PR scopes
verification to the affected module.

Part 1 of 4 from #22.

Author: hyperpolymath

crates/my-lang/lib/common/math.rs

@@ -265,7 +265,7 @@ mod tests {
     fn test_abs() {
         assert_eq!(abs_int(-5), 5);
         assert_eq!(abs_int(5), 5);
-        assert!((abs_float(-3.14) - 3.14).abs() < f64::EPSILON);
+        assert!((abs_float(-2.5) - 2.5).abs() < f64::EPSILON);
     }
 
     #[test]

crates/my-lang/lib/common/string.rs

@@ -297,7 +297,7 @@ mod tests {
     #[test]
     fn test_parse() {
         assert_eq!(parse_int("42"), Some(42));
-        assert_eq!(parse_float("3.14"), Some(3.14));
+        assert_eq!(parse_float("2.5"), Some(2.5));
     }
 
     #[test]

crates/my-lang/lib/common/types.rs

@@ -294,14 +294,14 @@ mod tests {
 
     #[test]
     fn test_float_conversions() {
-        assert_eq!(str_to_float("3.14"), Some(3.14));
+        assert_eq!(str_to_float("2.5"), Some(2.5));
         assert_eq!(int_to_float(42), 42.0);
     }
 
     #[test]
     fn test_str_conversions() {
         assert_eq!(int_to_str(42), "42");
-        assert_eq!(float_to_str(3.14), "3.14");
+        assert_eq!(float_to_str(2.5), "2.5");
         assert_eq!(bool_to_str(true), "true");
     }