Unknown column names resolve to NULL instead of erroring, turning typos into empty results

[mp-grind]

Summary

Referencing a column that does not exist never fails. Depending on where the name
appears, the query returns nulls, returns nothing, or ignores the clause — but it
always exits 0. Unknown table names are rejected properly, so the strictness is
there for tables and missing for columns.

Since the documents / blocks schema isn't something a user can introspect
before writing a query, a misremembered column name is the most likely mistake to
make against this database, and it's currently the one with the quietest failure.

Reproduction

mkdir -p /tmp/mqbug/docs && cd /tmp/mqbug
printf '# Alpha\n\nIntro paragraph.\n\n## Install\n\nRun the installer.\n' > docs/a.md
printf '# Beta\n\nAnother paragraph.\n\n## Usage\n\nUse it well.\n'       > docs/b.md
mq-db index docs -o s.mq-db

In SELECT — column appears, filled with null

$ mq-db sql -d s.mq-db -F json "SELECT path, no_such_column FROM documents"
[{"path":"docs/b.md","no_such_column":null},{"path":"docs/a.md","no_such_column":null}]

In WHERE — predicate silently matches nothing

$ mq-db sql -d s.mq-db -F json "SELECT path FROM documents WHERE no_such_column = 'x'"
[]

This is the damaging one. [] is indistinguishable from a correct empty result,
so a typo reads as "no documents match" — a plausible answer a caller will
believe.

In ORDER BY — clause silently ignored

$ mq-db sql -d s.mq-db -F json "SELECT path FROM documents ORDER BY no_such_column"
[{"path":"docs/b.md"},{"path":"docs/a.md"}]

Control: unknown tables are rejected

$ mq-db sql -d s.mq-db "SELECT * FROM no_such_table"
Error: SQL execution error: unknown table: no_such_table

Expected

An error naming the unresolvable column, in all three positions — matching the
existing unknown-table behavior. Something like:

Error: SQL execution error: unknown column: no_such_column

A "did you mean" suggestion would be a nice touch, since mq already does this
(no function, selector, or module named 'nodes' — did you mean 'nones'?), but
the plain error is the part that matters.

Environment

• mq-db 0.2.1, installed via cargo install
• macOS 26.6.2 (build 25G83), arm64