fix: don't prefill or type widgets from i2's NotSet signature default

[thorwhalen]

What

Step 2/3 of the re-land plan in i2mint/i2#48, for streamlitfront: don't treat i2's NotSet sentinel as a real default.

• New streamlitfront.util.signature_defaults(sig): sig.defaults without the params whose default is NotSet.
• Used by the legacy spec builders that read sig.defaults directly: base.infer_type, base.get_func_args_specs, page_funcs.get_func_args_specs, page_funcs.special_get_func_args_specs, page_funcs.get_func_elements_commands.

The current mk_app path builds widgets through front's elements and pydantic helpers. It is fixed in i2mint/front#34, and streamlitfront's own elements never read obj.default. The independent review of that PR found this legacy path (dispatch_funcs → SimplePageFunc) as the one remaining place where the sentinel would become a widget's value= or its inferred type.

No-op against the current i2: no current signature carries NotSet.

i2 compatibility (decision)

Uses i2.is_not_set (i2 >= 0.1.74, i2mint/i2#91) when available, otherwise falls back on i2.deco.NotSet (present since 2023). The i2 requirement is unchanged.

Tests

streamlitfront/tests/test_not_set_defaults.py: for a NotSet-defaulted foo, infer_type and all three spec builders give the same result as for the plain foo, and there is no value= prefill. Both tests fail without the fix. There is also a doctest on signature_defaults.

Gate

• pytest --doctest-modules streamlitfront (py3.12, scrap/examples and the two selenium-only modules ignored because selenium is not installed here): 18 passed with i2 master and with i2 0.1.73 (fallback path). Master: 16 passed.
• ruff check / ruff format --check on changed files: clean.
• Dependents: extrude, know. know cannot be installed on this box (pyaudio needs system portaudio). extrude is tested below.
• Hosted CI: see checks.

🤖 Generated with Claude Code

[thorwhalen]

Independent refute-review (sub-agent): no blocking bug. Both tests fail without the fix. Normal defaults (None, 0, lists, tuples) give unchanged specs, and no other place in streamlitfront reads signature defaults (pydantic_widgets reads front's model fields).

Its findings, and what I did about them:

• front floor (medium): the mk_app / pydantic path is only NotSet-safe with a front release that contains fix: treat i2's NotSet signature default as required / no default front#34. I did not add a front>= floor here. It is not needed while FuncFactory signatures carry no NotSet, i.e. until Using a NotSet sentinel to solve make FuncFactory instances' signature more truthful about argument requirements. i2#48 step 3 lands, and then only for users who opt in. It is listed in Using a NotSet sentinel to solve make FuncFactory instances' signature more truthful about argument requirements. i2#48 as a precondition for flipping the default.
• page_funcs.get_func_elements_commands was already broken before this PR: it calls _get_dflt_element_factory_for_annot(inferred_type), which takes no arguments. The edit there only keeps the five legacy readers consistent. It is not a tested fix.
• There is no streamlitfront-level test of mk_app with NotSet. That coverage lives in front's tests.