feat: read build-details.json (PEP 739) for version detection (#452)

Fixes #449.

Python 3.14+ ships
[`build-details.json`](https://packaging.python.org/en/latest/specifications/build-details/)
(PEP 739) in the platform-independent stdlib directory. Read it as a
faster, more authoritative source than parsing `patchlevel.h` headers,
slotting it between `pyvenv.cfg` and the existing header-based fallback
in the version detection chain.

### Changes

- New `pet_python_utils::build_details` module — finds and parses
`build-details.json` from a sys.prefix-like path. Handles both Unix
(`<prefix>/lib/python<X.Y>[t]/`) and Windows (`<prefix>/Lib/`) layouts.
- New `version::from_build_details(prefix)` and updated chain in
`version::from_prefix`: `pyvenv.cfg → build-details → headers`.
- `version::from_creator_for_virtual_env` now also tries
`build-details.json` (with a `(major, minor)` hint from `pyvenv.cfg`) on
the creator's sys_root before scanning headers.
- Schema validation: only `1.x` accepted; unknown `releaselevel` or read
failures emit warnings.
- Stdlib directory matching uses a strict regex
(`^(python|pypy)\d+\.\d+t?$`) to avoid false positives like `python3`
(versionless) or `python-config`.
- Unit tests in the module + integration tests with realistic JSON
fixtures, including chain-priority assertions (`pyvenv.cfg` beats
`build-details`, `build-details` beats `headers`).

Author: karthiknadig

Cargo.lock

@@ -18,6 +18,9 @@ serde_json = "1.0.93"
 sha2 = "0.10.6"
 env_logger = "0.10.2"
 
+[dev-dependencies]
+tempfile = "3.10"
+
 [features]
 ci = []
 ci-jupyter-container = []