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.

CHANGELOG.md

@@ -57,6 +57,17 @@ callers).
 
 ### Changed
 
+- **`ParsedRelease.tech_string` is now a derived `@property`**
+  (`alfred/domain/release/value_objects.py`). It computes
+  `quality.source.codec` joined by dots on every access, so it stays in
+  sync with the underlying fields by construction. The stored field is
+  gone from the dataclass, the dict returned by `assemble()` no longer
+  carries the key, `parse_release`'s malformed-name fallback drops the
+  `tech_string=""` kwarg, and `enrich_from_probe` no longer re-derives
+  it after filling `quality`/`source`/`codec`. Closes the
+  parser/enrichment double-source-of-truth that `e79ca46` had to fix
+  reactively. The fixtures runner now injects `tech_string` alongside
+  `is_season_pack` since `asdict()` skips properties.
 - **`RuleScope.level` is now an enum (`RuleScopeLevel`).** The set of
   valid levels (global, release_group, movie, show, season, episode)
   was documented only in a docstring comment and validated nowhere.