Matrix: typing-surface infrastructure + insert Form A coverage

Per docs/handoff/20260515-handoff-12.md §1. Systematic per-position
coverage of (state, hint, completion, parse_result) across canonical
schema shapes; submodule per command family. Insert Form A covers 23
cursor positions across serial-PK, text-PK, multi-table, and
every-Type schemas. Both bugs fixed in the previous commit were
surfaced by these tests.

Shared helpers under tests/typing_surface/mod.rs: 5 canonical schema
shapes, assess() helper, property-assertion shortcuts, and a snap!
macro that wraps insta with a stable per-cell suffix.

859 -> 885 tests passing; 1 ignored (pre-existing doc-test).

tests/typing_surface/update_all_rows.rs

@@ -0,0 +1 @@
+//! Submodule stub — populated in subsequent tasks.