feat(expression): rebuild the column layer - typed columns, node formatters, and the bare-Column reference

.claude/rules/README.md

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+# Rules
+
+How this codebase is built, and what has been tried and rejected. Each file
+stands alone; read the one that concerns the change in front of you.
+
+| Rule | Covers |
+|---|---|
+| [expression-direction.md](expression-direction.md) | `expr -> format` and why the reverse direction is where injection and unreviewable inference live. What a factory signature may accept. |
+| [column-and-result-types.md](column-and-result-types.md) | Designing column classes; the core / backend / user layering; stating a result type the source cannot settle, and why it is a mixin rather than a keyword. |
+| [operation-contracts.md](operation-contracts.md) | What each operation promises once the ten-backend survey measured it: the string boundary refusals, the rendering-name contracts, the temporal result types, the value-layer echo, and where version gates refuse. |
+| [static-analysis.md](static-analysis.md) | Making a checker actually check: `py.typed`, a configuration that loads, and the defects that opt out of checking silently. |
+| [anti-patterns.md](anti-patterns.md) | The catalogue, with what each one cost. Read this before adding a runtime lookup, a coercion helper, or a `__getattr__`. |
+
+Two rules decide most of the rest:
+
+1. **Information flows one way**, from a typed object to SQL text. A formatter
+   needs an expression to have anything to format, so nothing may manufacture
+   the expression it is about to render.
+2. **A backend is entirely static.** An IDE and a type checker give a definite
+   answer. Nothing is worked out at run time, and nothing is left for the
+   caller to assert in a form a checker cannot read.
+
+A third applies to the checks themselves, and it is the one most often skipped:
+**a check that was not run cannot have passed.** The defects CI found in one
+afternoon — four version gates that could not say no, an argument inserted into
+the wrong call — were all in files that had been edited and not tested. Run the
+directory.