A vec2 argument type: two numbers validated against their declared ranges

[Jules-GP]

The server half of vec2 - two numbers set together, because they are one
position rather than two settings.

The client already renders it (a 2D pad plus two spin boxes, with drag, Ctrl for
a finer gesture, wheel and arrow keys) and nothing produced it. The generator
half is sadt-tools#24; this is what lets a schema declaring one load, and what
validates the value before run() sees it.

What it adds

• VEC2_TYPE and _coerce_vec2 in base.py: a JSON array or the
  comma-separated shorthand, exactly like the list and multichoice types.
• x_range/y_range on ArgSpec, and vec2 in the schema vocabulary.
• x_labels/y_labels, which name the two ends of an axis: "0.8" says nothing
  about where that is in a mouth, and "mid"/"out" does.

The ranges are not presentation

They are the one layout key that reaches validate(). A request that skips the
panel would otherwise place a patch corner off the arch and be answered with a
success, which is the failure this repository keeps finding in other forms. A
mirrored axis is still written by inverting the range (x_range: [15, -15] puts
15 at the left end), and validation compares against the min and max rather than
the declared order - a test pins that.

Verified

An annotation through describe.py, the generated schema through
_argument_spec, and validate() refusing y = 9 on an axis declared
[-5, 5], naming the axis and the value. Then FlexReg's real schema through the
client's formgen: five pads built from the schema alone, with the right
ranges and no client-side code for it.

315 server tests.

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