Pure, deterministic structural diff of two excessibility.digest/v1 maps.

Given a base and head digest (as produced by Excessibility.Digest, whether reloaded via Jason.decode!(..., keys: :atoms), decoded with string keys, or a raw Elixir map), diff/2 returns a stable, sorted structural delta. It emits no verdict, no severity beyond advisory, and no merge decision — only the observed structural differences plus measurement-scope notes.

The measurement-scope guard

A signal delta is asserted only when both digests measured that signal:

• Queries/plans are diffed only when both capture.ecto_configured are true. Otherwise the queries/plans lists stay empty and a coverage.notes entry explains that one side did not measure queries.
• Plans are diffed only when both capture.plan_capture modes match. A difference is scope-noted, never reported as a plan change.
• The plan-variant set per SQL fingerprint is bounded by the digest, so a side can drop structures it never emitted. When either side omitted variants the incompleteness is made explicit — a forced plans entry carrying per-side base_variants_omitted / head_variants_omitted counts, plus a coverage.notes entry — so a bounded comparison never renders "no change" for a structure it could not see (issue #183).
• Assign deltas are diffed only when the assign_sizes enricher ran on both sides; other enricher differences are scope-noted.
• A schema mismatch adds a prominent note.

This keeps compare honest: a change in what was measured can never masquerade as a real regression.

Key tolerance

Every field read goes through get/3, which tries the atom key then the string key, so atom-keyed and string-keyed digests behave identically.

Determinism

All output lists are sorted by a stable key (fingerprint, {view, callback}, or name) and all set math is order-independent, so the diff of a given pair is byte-for-byte reproducible regardless of input event order.