fix(pegboard): delete branch-backed sqlite on actor destroy

[SaiThanushreddy]

Description

Fixes #5561

Destroying a sqlite: remote actor cleared only the legacy actor-ID layout (meta_head/shard/delta/pidx via clear_v2_storage_for_destroy) and left the branch-backed layout BR/<branch_id>/... (DBPTR -> DatabaseBranchId) tombstoned. Branch refcount stayed 1 so depot GC never reclaimed it (branch_rows 39->43 in repro).

• delete branch-backed DBPTR database on actor destroy via depot::conveyer::branch::delete_database (tombstone + Add(refcount,-1)) so GC can reclaim
• make destroy idempotent on DatabaseNotFound (tracing::debug! per pegboard-envoy pattern)

Both pegboard/src/workflows/actor2/mod.rs:1238 ClearKv and pegboard/src/workflows/actor/destroy.rs:181 ClearKv now call clear_branch_storage_for_destroy outside the KV txn (branch delete has its own versionstamped txn). Legacy clear_v2 kept for pre-branch actors.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• cargo check -p pegboard --all-targets ok
• cargo test -p pegboard --test actor_sqlite_destroy -- --nocapture 6/6 ok:
  • actor_destroy_clears_branch_backed_database — refcount 1->0, list_databases []
  • actor_destroy_branch_is_idempotent — double delete no-op
  • actor_destroy_branch_noop_when_no_database — no branch/bucket no-op
  • actor_destroy_clears_both_layouts_like_workflow — legacy clear_v2 + branch delete wiring
  • 2 existing legacy tests still pass

Repro repro-actor-destroy-sqlite.sh now shows refcount 1->0, list_databases [] (branch BR/ rows remain until sweep_unreferenced_branch GC — expected logical delete).

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[SaiThanushreddy]

@claude review

[SaiThanushreddy]

@abcxff @MasterPtato