fix: single source of truth for version across crate and tests

Replace all hardcoded "1.1.0" version strings with env!("CARGO_PKG_VERSION")
in src/, so Cargo.toml is the sole source for the built binary. Tests
intentionally use a separate hardcoded constant in tests/common.rs to act
as a canary — they will fail on a version bump until manually updated.

- src/project/mod.rs: add WEEVIL_VERSION const, wire into Tera context,
  generated README, and .weevil-version marker
- tests/common.rs: new file, holds EXPECTED_VERSION for all test crates
- tests/{integration,project_lifecycle,unit/config_tests}.rs: pull from
  common instead of env! or inline literals

tests/integration.rs

@@ -2,6 +2,10 @@ use assert_cmd::prelude::*;
 use predicates::prelude::*;
 use std::process::Command;
 
+#[path = "common.rs"]
+mod common;
+use common::EXPECTED_VERSION;
+
 #[path = "integration/environment_tests.rs"]
 mod environment_tests;
 
@@ -25,7 +29,7 @@ fn test_version_command() {
     
     cmd.assert()
         .success()
-        .stdout(predicate::str::contains("1.0.0"));
+        .stdout(predicate::str::contains(EXPECTED_VERSION));
 }
 
 #[test]