[idea] Build loop: visual check per page during the build + state save-then-check explicitly; route MCP-owner asks upstream

[MendixMau]

The idea: two habits from the MCP arm are worth adopting in iterative-build-loop.md:

• A visual check of each page while it is being built, not after the person complains.
• An explicit save-then-check rule. The MCP arm lost time checking against a stale copy on disk: 229 errors stayed hidden until a real save. Say plainly that the toolkit gate always checks the saved file.

Also route the asks meant for the MCP/Concord owners through upstream-feedback.md:

• write support for security, roles, home pages, snippets, scheduled events, mappings, published REST and widget properties;
• atomic batch writes;
• auto-save before check.

Field evidence: contrib/inbox/2026-09-29-mcp-arm-observations.md (#165).