Fold the integration fixture database into Store

[SaladDay]

Makes *Store the only integration test fixture value. Follow-up to #449, from minor 3 of its review.

• Deleted fixtureDB, which duplicated Store's pool, key and placement rules, along with the new*TestStoreDB wrappers that paired the two.
• Placement rules now come from one place, Store.placement. Before, fixtureSessions built its own rules from a separate public URL, so a test could set one source and silently get the other.
• One sessionpg constructor (sessionAdapter) and one Session service constructor (newSessionService). publicHandler uses a single Session service for Sessions and SessionCreation, as cmd/server does.
• Test-only, mechanical edits. No assertion changed, and no production code changed.

Checks:

• go build ./..., go vet ./services/core/..., gofmt and check-names.
• All three integration shards with the database: 411 tests, 379 pass, 32 skip (official SDK, native daemon and operator binary only), 0 fail.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}