Fix test_ch_funcs fossil of the double_up_as_factory keyword bug

[thorwhalen]

Land this BEFORE (or together with) i2mint/i2#82

i2mint/i2#82 fixes double_up_as_factory so that a decorator given its wrapped object
by keyword decorates instead of silently returning a functools.partial factory.

meshed.dag.ch_funcs is built with double_up_as_factory, and one meshed test was
written against the broken behaviour. That test goes pass -> fail the moment i2#82
lands. This PR removes the fossil, so it should land first (or at the same time).

It is written to be safe to merge now, before i2#82 ships — see "Ordering" below.

The fossil

ch_funcs's first parameter is func_nodes. Before i2#82:

ch_funcs(func_nodes=nodes, func_mapping=mapping)   # -> functools.partial  (wrong)
ch_funcs(nodes, func_mapping=mapping)              # -> DAG                (right)

test_ch_funcs_no_change used the keyword form and then added a trailing () to turn
the unexpected factory into the DAG it actually wanted:

new_dag = ch_funcs(func_nodes=nodes, func_mapping=dummy_mapping)
new_nodes = new_dag().func_nodes   # the () only made sense because of the bug

After i2#82, ch_funcs(func_nodes=...) returns the DAG directly, so that trailing ()
calls the DAG, which raises:

TypeError: missing a required argument: 'a'

Verified A/B against a local i2 checkout: meshed/tests/test_ch_funcs.py gives
3 passed with i2 master and 1 failed, 2 passed with the i2#82 branch, deterministically.

The fix

• test_ch_funcs_no_change now passes the func nodes positionally and reads
  new_dag.func_nodes directly. That is what the test always meant to assert
  ("an identity mapping changes nothing"), and it behaves identically on old and new i2,
  so this test keeps covering its intent regardless of which i2 is installed.
• New test_ch_funcs_takes_func_nodes_by_keyword pins that ch_funcs(func_nodes=...)
  returns a DAG and not a partial, and that it agrees with the positional form —
  so the fossil cannot silently come back.
• Added the missing module docstring.

Ordering / why this is safe to merge before i2#82

The new pin asserts behaviour that only the fixed i2 has. Rather than pin an i2 version
that does not exist yet, it feature-detects (_wrapped_by_keyword_is_supported()
builds a throwaway double_up_as_factory decorator and checks it). So:

installed i2	pin
current release (pre-#82)	skipped, with a reason naming i2#82
with i2#82	runs and enforces

The pin is not vacuous: with the guard temporarily removed and pre-#82 i2 installed it
fails with AssertionError: ch_funcs wrongly returned a factory.

Test results

meshed is already red on master — 3 pre-existing doctest failures unrelated to this
change (meshed/dag.py, meshed/makers.py, meshed/scrap/cached_dag.py). Those are
untouched here. Full suite (pytest --doctest-modules):

i2	meshed	result
master	before this PR	3 failed, 157 passed, 1 skipped
i2#82 branch	before this PR	4 failed, 156 passed, 1 skipped <- the regression
master	this PR	3 failed, 157 passed, 2 skipped (pin skipped)
i2#82 branch	this PR	3 failed, 158 passed, 1 skipped (pin runs)

Same 3 pre-existing failures in every case: the baseline is not worsened, and the
regression is gone.

Ecosystem survey

I searched the whole local package ecosystem for other call sites of this shape — passing
the wrapped object by keyword to a double_up_as_factory-built decorator and then calling
the result — both statically (every such decorator in i2, meshed, front, larder, py2http,
plunk, wip_qh) and dynamically (a temporary probe in double_up_as_factory logging every
call that takes the new code path, run across all 46 i2 dependents' suites).

This test was the only one in executed code. A re-review of the survey turned up a second occurrence of the same fossil in meshed/scrap/notebook.ipynb — a 2023 scratch notebook whose saved output shows the buggy functools.partial directly. That one is dead code (scrap/ is in this repo's CI paths-to-ignore, and no plugin collects notebooks), so it changes no test result; it is left untouched rather than edited, because rewriting its source without re-running it would desynchronise the cells from their stored outputs. Details are in the i2#82 thread.

https://claude.ai/code/session_01Kug7UUbVeCQgruvNXUq63c