`LEFT JOIN` never emits unmatched left rows — behaves as `INNER JOIN`

[mp-grind]

Summary

LEFT JOIN drops left-hand rows that find no partner instead of emitting them
with NULLs. The result is identical to the equivalent INNER JOIN. This makes
anti-joins (LEFT JOIN ... WHERE right.col IS NULL) impossible to express, which
is the natural way to ask the most common corpus question there is: which
documents are missing X?

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

docs/a.md has an Install heading; docs/b.md does not.

$ mq-db sql -d s.mq-db -F json \
  "SELECT d.path, b.id FROM documents d JOIN blocks b ON b.document_id = d.id AND b.content = 'Install'"
[{"path":"docs/a.md","id":6}]

$ mq-db sql -d s.mq-db -F json \
  "SELECT d.path, b.id FROM documents d LEFT JOIN blocks b ON b.document_id = d.id AND b.content = 'Install'"
[{"path":"docs/a.md","id":6}]

Expected from the LEFT JOIN: two rows, the second being
{"path":"docs/b.md","id":null}. Actual: byte-identical to the INNER JOIN.

Consequently the anti-join returns nothing:

$ mq-db sql -d s.mq-db -F json \
  "SELECT d.path FROM documents d LEFT JOIN blocks b ON b.document_id = d.id AND b.content = 'Install' WHERE b.id IS NULL"
[]

Expected docs/b.md.

It isn't IS NULL, and it isn't join resolution

IS NULL / IS NOT NULL evaluate correctly on ordinary columns:

$ mq-db sql -d s.mq-db -F json "SELECT path FROM documents WHERE title IS NOT NULL"
[{"path":"docs/b.md"},{"path":"docs/a.md"}]

And the join itself resolves fine when every left row happens to match — all 8
pairs come back:

$ mq-db sql -d s.mq-db -F json "SELECT d.path, b.id FROM documents d LEFT JOIN blocks b ON b.document_id = d.id"
[{"path":"docs/b.md","id":0}, ... 8 rows ... ,{"path":"docs/a.md","id":7}]

So the missing piece is specifically the generation of NULL-extended rows for
unmatched left tuples.

The same happens when the right side is a CTE, so it isn't specific to base
tables:

$ mq-db sql -d s.mq-db -F json \
  "WITH h AS (SELECT document_id FROM blocks WHERE content = 'Install') SELECT d.path FROM documents d LEFT JOIN h ON h.document_id = d.id WHERE h.document_id IS NULL"
[]

(The INNER JOIN form of that CTE query works correctly and returns docs/a.md.)

EXPLAIN

$ mq-db sql -d s.mq-db "EXPLAIN SELECT d.path, b.id FROM documents d LEFT JOIN blocks b ON b.document_id = d.id AND b.content = 'Install'"
│ query:from    │ documents (documents)                                        │
│ query:where   │ none — full scan                                             │
│ query:join[0] │ blocks: hash join on b.document_id = d.id — join partner always full-scanned │

The plan records only hash join, with no left/outer marker — the join type
appears not to survive into the plan. (Minor, separate observation: the extra
AND b.content = 'Install' conjunct in the ON clause is absent from the plan
detail too, though it clearly is applied to the results.)

Environment

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