[thekevinbot]

Fixes #1054. Step under #1053 (census/plan) and #1020 (tracker); lane C, the rung after #1042 / PR #1051 (16 -> 12).

Makes internals/checks/src clean at one_function_per_file max_lines = 9 by splitting files only.

No config change. internals/checks/testing-conventions.toml still reads max_lines = 15. Per invariant 1 in #1053, split PRs never touch a max_lines line -- the ceiling drops in its own PR once every lane is clean at 9, so five concurrent lanes never contend for the same config line.

Work list, measured

Throwaway shallow clone of origin/main at b2c4651, internals/checks/testing-conventions.toml rewritten to max_lines = 9, nothing committed. npx -y testing-conventions@0.0.111 because npm latest is what the reusable workflow runs; the PyPI CLI is a separately versioned distribution (0.0.104) and does not reproduce the gate.

$ npx -y testing-conventions@0.0.111 unit one-function-per-file \
    --language python --config internals/checks/testing-conventions.toml internals/checks/src
artifact_completeness/gate.py:92:    `run` runs 20 lines, and `missing` already holds this file
declared_deps/gate.py:129:           `run` runs 20 lines, and `undeclared` already holds this file
platforms_mirror/decide.py:108:      `field_problems` runs 12 lines, and `missing_rows` already holds this file
platforms_mirror/decide.py:124:      `problems` runs 10 lines, and `missing_rows` already holds this file
platforms_mirror/python_table.py:48: `_table_elements` runs 10 lines, and `_dataclass_fields` already holds this file
error: 5 function(s) sharing a file with another function over the 9-line threshold

Five, matching #1053's ladder. With this branch and the same temporary max_lines = 9 the run is silent.

Moves

Function	From	To
run	artifact_completeness/gate.py	artifact_completeness/run.py
run	declared_deps/gate.py	declared_deps/run.py
field_problems	platforms_mirror/decide.py	platforms_mirror/field_problems.py
problems	platforms_mirror/decide.py	platforms_mirror/problems.py
_table_elements -> table_elements	platforms_mirror/python_table.py	platforms_mirror/table_elements.py

Colocated unit tests move with the code they cover rather than being duplicated. Call sites switch through relative imports: cli.py in both check packages, and platforms_mirror/gate.py (from .decide import problems -> from .problems import problems). Two integration tests that drove the check through gate.run now import from run.

Two notes on the last row. _table_elements takes its only caller _assigned_names with it -- leaving that helper in python_table.py would have made the two modules import each other, the cycle hazard #1053 calls out. And it loses its leading underscore, since a module existing only to hold a private function reads badly and the package already names modules after their function (python_table, row, typescript_table).

No constant moved, so none of #1053's relocated-constant mutation survivors apply. wheel_extension_load -> npm_binary_extension_load, the tree's one cross-package import, is untouched at this threshold.

Red/green

Does not apply. These are pure moves with no behavior change: no new observable behavior to capture in a red test, and the tests that cover the moved functions move with them and stay green throughout. Section "Removals are exempt" in AGENTS.md is the nearest analogue; the pushed-RED-CI gate has nothing to assert here.

Gates

just preflight -- 0 failing pairs, 3 skipped (the three packaging pairs, which need a built artifact). Notable results in the internals-checks lane:

• one-function-per-file clean at the committed ceiling, and clean at 9 with the config temporarily rewritten.
• unit-coverage 100%, unchanged.
• unit mutation: 63 mutants tested, no survivors.
• colocated-test (including the PR-only co-change check) clean.
• unit lint isolation clean -- every new test file imports exactly one source module, the invariant the package already held.
• internals/checks integration suite: 43 passed. E2E suite: 24 passed.

Manual run

Exercised all three affected CLIs against the real repo, output unchanged from main:

• dirsql-checks platforms-mirror -> ok platforms-mirror: ... mirrors the shared fields ... across 5 published target(s)., exit 0
• dirsql-checks declared-deps internals/checks/src -> silent, exit 0
• dirsql-checks artifact-completeness --dist-dir <scratch dist with one staged target> --config putitoutthere.toml -> the skip line for dirsql-py, four incomplete artifact -- lines, the summary naming the likely cause, exit 1

E2E Verification

• Ran e2e suites locally for every affected SDK
• Python SDK e2e: N/A -- no packages/python change
• TypeScript SDK e2e: N/A -- no packages/ts change
• Rust core e2e: N/A -- no packages/rust change
• internals/checks/e2e-attestations/claude-1054-checks-ofpf9.json receipt written (just e2e-attest-internals-checks); git show --stat on the attest commit confirms it adds one file and deletes no sibling receipt (e2e attest silently deletes other branches' attestation receipts #1041)
• Command(s) run: just test-e2e-internals-checks, just test-integration-internals-checks, just e2e-attest-internals-checks, just preflight
• Result summary: e2e 24 passed, integration 43 passed, preflight 0 failing / 3 skipped

Changelog / Migrations

Neither is required. changelog_gate/decide.py's _ROOTS = ("packages", "plugins") puts internals/** outside the gate; no public API changes, so no migration fragment and no PARITY.md change.