refactor(release): make tech_string a derived property

ParsedRelease.tech_string was a stored str field re-computed in two
places (assemble() at parse time, enrich_from_probe() after the probe).
The second site was a reactive fix (e79ca46) for filename builders that
saw a stale value. Turn it into an @property so it stays in sync with
quality/source/codec by construction.

- Drop the field from the dataclass + the key from assemble()'s dict.
- Drop tech_string="" from parse_release's malformed-name fallback.
- Drop the manual recomputation at the end of enrich_from_probe.
- Inject the property into asdict() result in the fixtures runner
  (same treatment as is_season_pack).
- Update tests that passed tech_string= to the constructor; rewrite the
  TestTechString case that mutated p.tech_string manually.

tests/domain/test_release_fixtures.py

@@ -44,8 +44,10 @@ def test_parse_matches_fixture(fixture: ReleaseFixture, tmp_path) -> None:
 
     parsed, _report = parse_release(fixture.release_name, _KB)
     result = asdict(parsed)
-    # ``is_season_pack`` is a @property — asdict() does not include it.
+    # ``is_season_pack`` and ``tech_string`` are @property values —
+    # ``asdict()`` does not include them.
     result["is_season_pack"] = parsed.is_season_pack
+    result["tech_string"] = parsed.tech_string
 
     for field, expected in fixture.expected_parsed.items():
         assert field in result, (