knob: expose zero_direction and dead_zone_degrees accessors

[RobLoach]

Problem

nk_console_knob_data (nuklear_console_knob.h:11-12) has two customization fields:

typedef struct nk_console_knob_data {
    nk_console_property_data property; /* Inherited from property */
    enum nk_heading zero_direction;
    float dead_zone_degrees;
} nk_console_knob_data;

They are hardcoded to NK_DOWN / 60.0f at creation and consumed by the render path in nuklear_console_property.h:209,213, but there is no public API to read or change them — grep shows only internal use. This is inconsistent with sibling widgets that expose accessors for their data fields (button: nk_console_button_get/set_symbol, nk_console_button_get/set_image; image: nk_console_image_get/set_color).

Suggested Implementation

Add to nuklear_console_knob.h, following the button/image accessor pattern (NULL-safe, Doxygen-commented):

NK_API enum nk_heading nk_console_knob_get_zero_direction(nk_console* knob);
NK_API void nk_console_knob_set_zero_direction(nk_console* knob, enum nk_heading zero_direction);
NK_API float nk_console_knob_get_dead_zone_degrees(nk_console* knob);
NK_API void nk_console_knob_set_dead_zone_degrees(nk_console* knob, float dead_zone_degrees);

Guard against NULL widget/data and wrong widget type, as the existing accessors do.

QA

• Unit tests in test/nuklear_console_test.c: defaults are NK_DOWN/60.0f; set/get round-trips; NULL widget doesn't crash.
• Add the new functions to the README API list (keep in sync with docs: sync README API signatures and add missing list_view prototype #278).

Context

• Priority 5 of the 2026-07-11 planning session.
• Knob widget originated in Widget: Knob #105; accessor-pattern precedent in nuklear_console_button.h / nuklear_console_image.h.
• Soft ordering: land after docs: sync README API signatures and add missing list_view prototype #278 so the README API sweep there doesn't race this addition.