…undefined/null as equal

Two audit-writer diff bugs surfaced by the objectui record History tab
(gantt QA report showed "紧前依赖(渲染用): [id] → —" on every drag):

1. The beforeUpdate snapshot is read back through the query path, which
   computes formula/summary/rollup/autonumber fields; ctx.result is the raw
   write result, which does not. diff() therefore recorded a phantom
   "value → null" change for every computed field on every update. Computed
   fields are now excluded from the diff via the engine schema (their
   changes are implied by their source fields); an update touching only
   computed fields no longer writes an audit/activity row at all.

2. safeStringify(undefined) returns undefined (JSON.stringify contract),
   not a string, so a key absent on one side compared unequal to an
   explicit null on the other and wrote a noise row with old=new=null.
   Values are normalized with `?? null` before comparison; a real
   value → null transition is still recorded (covered by a guard test).

Verified against a live showcase stack: updating showcase_project.health
now writes old_value {"health":"green"} / new_value {"health":"yellow"}
with no task_count/total_estimate phantoms, and the three new regression
tests fail against the pre-fix writer.

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