[EPAC-2303]: Relocate ComputeBillVersionDiff policy to bills-indexer use-case layer

[riddim-developer-bot]

Scope

Relocates the clause-diff application policy out of the LEGISinfo source adapter and into the bills-indexer use-case layer, so the boundary matches the documented ComputeBillVersionDiff use case (EPAC-2303; found during EPAC-2293 verification).

The diff algorithm (DiffClauses + buildDiffs) operates purely on domain.VersionSection → domain.BillClauseDiff and is independent of the LEGISinfo wire format, so it belongs inward in the application layer, not in a source-named adapter. This is a behavior-preserving move — the artifact output is unchanged.

What changed

• DiffClauses + buildDiffs moved into internal/usecase/compute_bill_version_diff.go as exported ComputeBillVersionDiff + DiffClauses, importing only the standard library plus the local domain package.
• Composition: IngestBills.Execute now computes diffs over the fetched batch (after the adapter has parsed sections) and before the SQLite write. The LEGISinfo adapter keeps XML/HTML retrieval and parseBillXML section extraction only.
• Output is unchanged for existing fixtures: bill.Number, bill.Versions, and bill.SourceURL feed the policy with exactly the values toDomain passed to buildDiffs before, so diff/clause IDs and rows are byte-identical.
• Tests: the diff unit tests moved to the use-case package; parseBillXML's test stays in the adapter (parse_test.go). Added TestExecuteComputesVersionDiffsDuringIngest, which proves the use case composes the diff during ingestion (not just that the standalone policy works).
• Catalog: ComputeBillVersionDiff now lists real implementation paths and the accurate port/adapter split (pure domain→domain policy; no port of its own); IngestBillVersionText gains its missing Current implementation: block.

Acceptance criteria

• DiffClauses/buildDiffs moved into internal/usecase/ importing only stdlib + domain.
• XML/HTML retrieval and parseBillXML remain in the legisinfo adapter.
• Pipeline composes parsing (adapter) then diff (use case); domain.Batch output preserved for existing fixtures.
• Diff unit tests moved with the policy and pass against the use-case package.
• No file under internal/usecase/ imports net/http, encoding/xml, or the legisinfo adapter package.
• Catalog ComputeBillVersionDiff updated; IngestBillVersionText gains its Current implementation: block.

Bugfix SPEC

• N/A — architecture refactor from a Linear issue, not a bug fix.

Testing notes

• Automated tests run: go test ./backend/bills-indexer/... — all pass. go vet ./... clean.
• Boundary check: grep -rE '"net/http"|"encoding/xml"|adapter/legisinfo' internal/usecase/ returns nothing.
• Manual verification: N/A — internal backend refactor with no UI surface; bills artifact output and CLI run logs are unchanged. (No simulator screenshot applicable.)

Screenshots

N/A — no UI change.

Related issue

• Closes: EPAC-2303