fix: treat i2's NotSet signature default as no default

[thorwhalen]

What

Step 2/3 of the re-land plan in i2mint/i2#48, for dagapp: i2's NotSet sentinel in a signature means "no default".

• get_root_values: NotSet defaults are ignored, so the root gets the annotation's DFLT_VALS entry or 0.0, exactly as a param with no default does. Before, the sentinel itself became the widget's initial value.
• _compute_node_value: a param missing from session state is only skipped (left to its default) if it has a real default (_has_default). A NotSet one behaves like a required param, as today.

No-op against the current i2: no current signature carries NotSet. There are no dependents in the fleet.

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

• dagapp/tests/test_not_set_defaults.py: get_root_values for a DAG over a NotSet-defaulted function equals the plain one's, with no sentinel. Fails without the fix.
• Doctest on _has_default.

Gate

• pytest --doctest-modules dagapp (examples ignored), py3.12: 2 passed with i2 master and with i2 0.1.73 (fallback path). Master collects no tests.
• ruff check / ruff format --check: clean.
• Hosted CI: see checks.

🤖 Generated with Claude Code

[thorwhalen]

Independent refute-review (sub-agent): dagapp itself is correct. The only places that read defaults are get_root_values and _compute_node_value. For normal params, _compute_node_value behaves as before (default None/0: skip; no default: KeyError; *args/**kwargs: unchanged). Both test runs pass (i2 master and i2 0.1.73). There is no test for the _compute_node_value change, which only differs for NotSet.

It also refuted my assumption that meshed needed no change. meshed sorted NotSet-defaulted params after required ones, which silently reorders a DAG's positional params. It also raised on merging a NotSet param with a same-named required one. That is fixed separately in i2mint/meshed#82. dagapp builds its roots from dag.sig, so it benefits from that fix, but this PR does not depend on it.