[thorwhalen]

Closes #161

Adds dol.on_overwrite(store, *, policy="warn"|"raise"|"overwrite"|callable, same=..., stacklevel=3): an overwrite policy for stores (class, instance or decorator factory), built on a wrap_kvs preset. Writing a same value is silent (default same: identity, then ==, and an ambiguous or raising == counts as different). Additive API only.

Review (two rounds, fresh Opus agents). Round 1 verdict "merge after fixes": default == crashed on array-like values; hardcoded stacklevel; brand's Registry couldn't adopt it (no custom message/exception). Applied: safe default same, stacklevel kwarg, callable policy as strategy, shared _existing_if_different helper with mk_confirm_overwrite_preset (behaviour unchanged), policy="overwrite" returns the store unwrapped, OVERWRITE_POLICIES/preset factory made private, docstring notes (per-write read, not thread-atomic, delete-then-write bypasses "raise"). Round 2: OK to land, no blockers; two non-blocking nits left (redundant policy re-resolve, one duplicated test).

Tests: full pytest 832 passed, 2 skipped locally.