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_matrix.rs

@@ -0,0 +1,19 @@
+//! Typing-surface matrix — systematic per-position coverage of
+//! what the user sees while typing a command (state, hint,
+//! completion candidates, parse outcome), against canonical
+//! schema shapes.
+//!
+//! Per `docs/handoff/20260515-handoff-12.md` §1. The matrix exists
+//! to catch the class of bug where automated tests pass but real
+//! users hit issues at specific typing positions — every commit
+//! in handoff-12's session fixed a bug that the 800+ existing
+//! tests had missed. Each row in this matrix is one (input,
+//! cursor, schema) cell; failures here mean the typing surface
+//! drifted or regressed at that exact cell.
+//!
+//! Layout: helpers and per-command submodules live under
+//! `tests/typing_surface/`. Each submodule exercises one command
+//! family across the canonical schema shapes defined in
+//! `mod.rs`.
+
+mod typing_surface;