Refactor final4 paths to graph-derived structure

[mateu]

What

Derive Final Four and region-winner bracket paths from game-graph data instead of fixed game IDs.

Why

Hardcoded path IDs make bracket-year maintenance brittle and force structural assumptions into controller/validation code.

How

Added a Bracket::Service::BracketStructure service that infers championship, semifinal, and region-winner paths from game_graph + game_team_graph ancestry.
Wired Final4 controller stash loading and BracketValidator final-four scope checks to this service.
Added structure-focused tests and updated validator tests to assert behavior via derived game IDs.

Testing

• script/test-env.sh prove -lv t/bracket_structure.t t/bracket_validator.t
• script/test-env.sh prove -lr t

---

Quality Report

Changes: 5 files changed, 215 insertions(+), 27 deletions(-)

Code scan: clean

Tests: skipped

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[mateu]

Superseded by #29, which has now been squashed and merged. Closing this predecessor PR to keep the active history clean.

[mateu]

Superseded by merged #29.