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/common.rs

@@ -0,0 +1,3 @@
+// Intentionally hardcoded. When you bump the version in Cargo.toml,
+// tests will fail here until you update this to match.
+pub const EXPECTED_VERSION: &str = "1.1.0-beta.1";