feat(storage): opt-in auto-GC after storage-tail compaction

[tiensonqin]

Summary

Storage.collect_garbage was manual-only, so storage files only grew. This adds an opt-in auto-GC policy evaluated after every storage-tail compaction, with the default left at upstream's manual-only behavior.

API surface (smallest that covers all conn/db entry points): a storage_auto_gc : auto_gc option field on the storage record — set it via ?auto_gc on memory_storage, file_storage, Datascript_sqlite.storage, or the logseq_sqlite_storage.ml example storage. No signature changes on create_conn/init_db/conn_from_*/restore_conn/transact.

type auto_gc = { min_garbage : int; garbage_fraction : float }
val default_auto_gc : auto_gc (* { min_garbage = 1024; garbage_fraction = 0.5 } *)

Trigger points: after a compaction stores a fresh index snapshot — in Conn.transact/Conn.apply_report (via a new maybe_collect_garbage field on transact_context, keeping conn.ml policy-agnostic) and in the db-level persist_transact_tail — maybe_collect_garbage storage recomputes addresses unreachable from the new root and storage_deletes them when they are both >= min_garbage and >= garbage_fraction of all stored addresses. With storage_auto_gc = None (the default) it is a no-op; collect_garbage remains the manual path.

Also exported for explicit use: Storage.default_auto_gc, Storage.auto_collect_garbage (evaluate a policy once), Storage.maybe_collect_garbage (honor storage_auto_gc). Refactored collect_garbage onto shared live_address_set/unreachable_addresses helpers. Documented in impl/datascript.mli and docs/design.md.

Split out of #38 — independent of the storage_tail representation change there; both land cleanly in either order.

Tests

All 34 native suites pass. test_storage__test_auto_gc covers: always-tripping policy leaves only reachable addresses while default storage accumulates garbage across repeated compactions; a high min_garbage floor keeps storage manual; maybe_collect_garbage is a no-op without storage_auto_gc and honors it when set; default_auto_gc values.

Link to Devin session: https://app.devin.ai/sessions/eb0e07b23c794857b6836d8a143d1688
Open in Devin Desktop: https://app.devin.ai/desktop/session/eb0e07b23c794857b6836d8a143d1688?variant=devin
Requested by: @tiensonqin

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