fix(tests): per-package test databases + legitimate approval fixtures (issue #2 close)

Root causes of the remaining 5 CI PG failures (and the hidden flaky set):
1. All PG-integration packages shared ONE database under parallel package
   execution - migrations/seeds/truncates raced (nondeterministic failure
   sets, locally and in CI).
   Fix: RequireTestDatabaseURL/SkipIfNoTestDatabaseURL now derive a
   per-package database (<base>_<pkgHash>, advisory-lock-guarded
   CREATE); 9 raw-env straggler tests funnelled through the helpers;
   db-package tests call it unqualified (no import cycle).
2. Approval-chain fixtures were not production-shaped: freezeReviewSnapshot
   only honors head_snapshot_id, fake content hashes never froze a
   snapshot, so ApproverApprove rejected with 'approved requires frozen
   review_snapshot_id'. Errors were swallowed (_, _ =) hiding this.
   Fix (ownership-respecting, arch-enforced):
   - docrevision.SeedContentSnapshotForTest seeds the ledger tables it owns
   - docgov GovernanceEngine.FreezeHeadSnapshotForTest freezes head/review
     pointers on document_records (docgov-owned) and clears engine cache
   - all four integration fixtures use the pair; swallowed errors checked
3. Fixed-name fixtures collide once a package shares one DB (projects
   uq_projects_name_global): uniquified in projectinstance + server tests.
4. CAS/ctxmgr tests never bootstrapped migrations (relied on other packages
   having migrated the shared DB): explicit RunMigrationsOnceForTest.

Verified: full parallel short suite = 75 packages, 0 failures against an
isolated throwaway PG. CI skip list removed - short tests fully required.

internal/docgov/testsupport.go

@@ -0,0 +1,26 @@
+package docgov
+
+import (
+	"context"
+	"fmt"
+)
+
+// FreezeHeadSnapshotForTest 把文档 head/review 快照指针冻结到 snapID
+// （写本模块所有的 document_records），并清除引擎缓存使后续装载来自 PG。
+//
+// 与 docrevision.SeedContentSnapshotForTest 配合：账本 seed 内容快照（docrevision
+// 生产行为），治理侧冻结指针（本模块生产行为）。供集成测试构造合法批准链。
+func (e *GovernanceEngine) FreezeHeadSnapshotForTest(ctx context.Context, docID string, snapID int64) error {
+	if e == nil || e.pool == nil {
+		return fmt.Errorf("docgov: FreezeHeadSnapshotForTest requires a PG-backed engine")
+	}
+	if _, err := e.pool.Exec(ctx,
+		`UPDATE document_records SET review_snapshot_id=$1, head_snapshot_id=$1 WHERE doc_id=$2`,
+		snapID, docID); err != nil {
+		return fmt.Errorf("docgov: freeze head snapshot: %w", err)
+	}
+	e.memMu.Lock()
+	delete(e.memDocs, docID)
+	e.memMu.Unlock()
+	return nil
+}