fix(language): a file whose terms read each other's sums is refused at load

[FBumann]

Prompt: Then do 5 now.

Note

The following content was generated by AI.

A file whose terms read each other's sums loaded alone and failed only at merge. It is now refused at load, and the message names the sums the loop runs through.

What this changes

lower resolves every term of a file first. Then _loop follows each term's reads through the file's other terms. A term that reads its own sum, directly or through another term, is refused with Given expression 's1': its term 'a' reads 's1', the sum the term adds to, through 's2', so the sum would define itself. The direct self-read keeps its message, without the through clause.

Why

Finding 5 of the review of #763. One file with s1: {term: a}, s2: {term: b}, a: p + s2 and b: p + s1 passed to_spec. merge then failed with a circular expression reference cascade that named neither the file nor the rewrite. The file alone decides this, so the load decides it.

Method, gate output, alternatives

Reproduced on origin/main (9ace314) before the change: to_spec(f) loads, and merge([owner, f]) fails with Named expression 's1': named expression 'a' does not load….

Test first. two-terms-reading-each-other-s-sum in test_what_a_term_may_not_be_is_refused_at_load failed on the unfixed tree (DID NOT RAISE) and passes with the fix. test_a_term_may_read_another_sum_its_file_adds_to holds that a read of another sum without a loop back still loads.

Gates. pixi cannot be installed in this environment, so I ran the tools from a venv directly:

• ruff format and ruff check src tests tools (0.16.1): clean.
• pytest -q -n auto: 2545 passed, 52 skipped.
• pyrefly check (1.2.0): the same 14 errors as on main. All are missing pydantic/yaml stubs in the venv, and none are in lowering.py.
• I did not run lefthook, docs-build or compile-tex.

Not done. #763 moves this check into lowering._term under adds_to:. This PR fixes it on main first. #763 takes it when it merges main in. No docs page states the self-read rule, so no page changes.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AzXL2w5xdUm8FsYMNWf1KW

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