feat(fixtures): port vgi-accumulate as the end-to-end BoundStorage fixture

Port the standalone vgi-accumulate worker into
vgi/_test_fixtures/accumulate/ — accumulate / accumulate_read /
accumulate_clear table functions persisting named row collections through
FunctionStorage, scoped per ATTACH via a catalog-minted random attach id.
The fixture exercises every BoundStorage surface end-to-end: persistent
attach-scoped K/V segments (incl. ranged scans/deletes for TTL eviction),
atomic row counters, and execution-scoped staging logs for the buffering
Sink->Combine->Source handoff.

Hosted inside the consolidated vgi-fixture-worker via MetaWorker (and the
HTTP fixture server), like the schema_reconcile and projection_repro
catalogs — no new entry points or env vars. Driven by the new
test/sql/integration/accumulate/*.test sqllogictests in the C++ repo and
mirrored by tests/conformance/test_accumulate.py (satisfies the
directory-parity guard).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: rustyconover