docs(wiki): digest the mdl/executor findings into nine bug-pattern pages

[ako]

Step 3b — the pattern pages. docs-wiki/bug-patterns/ had three pages, all synthesised on 2026-05-24 from the ~24 findings that existed then. mdl/executor alone now holds 248, and no page mentioned the area at all.

205 of the 248 (83%) now belong to a named class, across nine new pages.

The classes

page	findings	thesis
describe-round-trip-gaps	83	DESCRIBE is a second implementation of MDL, written in the opposite direction and validated by nothing
integration-contract-drift	30	OData / REST / mappings — the one area where neither mxcli check nor mxbuild is an oracle
duplicate-resolver-drift	23	one question answered in two places, and the two disagree
silent-property-drop	22	a typo and a real-but-unrouted property fail identically
platform-semantics-gaps	18	legal MDL, illegal Mendix — rules the grammar cannot carry
check-mxbuild-drift	17	mxcli check is a model of mxbuild, and models drift both ways
rewrite-drops-unauthored-state	13	CREATE OR REPLACE losing what the statement did not mention
unloadable-model-writes	12	load failure ≠ validation failure — no CE code, whole project down
flow-graph-geometry	10	a microflow is a graph with coordinates, and mxcli generates both

The three worth reading first

DESCRIBE has no oracle. mxbuild never sees DESCRIBE output, so a describer that drops a property produces a valid model at 0 errors on both sides. The write path is checked; the read path is not. That is why the class accumulates, and why it bites hardest where the feature is used most — describe → edit → exec is mxcli's copy operation. Four shapes, by how long they survive: won't parse, silently drops, invents a clause nobody wrote, destroys structure (a list view's 4 specialization templates went to 0, mx check clean on both sides).

Load failure is not validation failure. No CE code, no document named, Studio Pro will not open the project. The recurring cause is a reference whose shape is wrong; the recurring remedy is refusal in both passes; and mx check at 0 errors is not evidence the project opens, because mxbuild's deserializer tolerates properties Studio Pro rejects.

One question, two answers is the most common structural cause and the one whose symptoms look least alike. A forward reference accepted by check and rejected by exec, a guard that is inert on one engine, a property read by one describer and not its twin — same defect, different clothes. The tell is that the fix for the reported instance is obviously incomplete.

A correction, made while syncing and recorded in the log

The silent-property-drop draft named mdl/executor/validate_widget_props.go — which does not exist — and claimed the scalable remedy is "derived, not enumerated". Verifying the sources: list caught it. The real validate_widgets.go shows both halves exist and for good reason: derived for pluggable widgets (.mpk keys minus mapped keys), and a deliberately generous, drift-tested union for built-ins, which have no .mpk to subtract from.

Declaring a source without reading it is exactly what the sources: discipline exists to prevent, and it produced a wrong claim on the first attempt. Every sources: path and every [[wiki-link]] in the second pass was verified before committing.

Also worth recording: several reports in this area were measured and closed as correct behaviour — quoting escapes MDL's parser keywords and not Mendix's platform-reserved member names; else on a type split really is the (empty) flow; create really is non-idempotent in the SQL sense the language is modelled on. Those needed a better message or a doc fix, not code.

Contract

Per maintain-wiki.md: every page carries title / category / last-synced / sources, opens with a Do not duplicate guardrail pointing at canonical homes, and is concept-first — no recipes, no CE tables. Seed table extended by nine rows; nine rows appended to SYNC_LOG.md.

What is left

43 executor findings (17%) are still unclassified — a long tail with no shared mechanism. Then the areas no page mentions at all: cmd/mxcli (95), mdl/backend (83), mdl/grammar (53), mdl/visitor (28).

🤖 Generated with Claude Code