Implement PDF forms support in pdfgen.c

[Copilot]

Summary

Adds interactive PDF forms (AcroForms) support to pdfgen, providing three new public API functions and all the infrastructure needed to produce valid, fillable PDF form fields.

New Public API

// Single-line text input field
int pdf_add_text_field(struct pdf_doc *pdf, struct pdf_object *page,
                       float x, float y, float width, float height,
                       const char *name, const char *value,
                       float font_size, uint32_t colour);

// Boolean checkbox
int pdf_add_checkbox(struct pdf_doc *pdf, struct pdf_object *page,
                     float x, float y, float width, float height,
                     const char *name, int checked);

// Radio button (grouped; call once per option)
int pdf_add_radio_button(struct pdf_doc *pdf, struct pdf_object *page,
                         float x, float y, float width, float height,
                         const char *radio_group_name,
                         const char *option_name, int selected);

A new enum (PDF_FIELD_TYPE_TEXT, PDF_FIELD_TYPE_CHECKBOX, PDF_FIELD_TYPE_RADIO, PDF_FIELD_TYPE_RADIO_BUTTON) is exposed in pdfgen.h.

Changes

pdfgen.c

• OBJ_form_field — new object type added to the object enum.
• form_field union member — stores field type, page, geometry, name/value strings, font properties, checked state, parent/kids pointers (for radio groups), and on/off appearance XObject references.
• pdf_object.form_font — new pdf_doc field caching the Helvetica font used for AcroForm /DR and text-field /DA strings.
• pdf_object_destroy — clears the kids flexarray for radio groups.
• pdf_save_object — new OBJ_form_field case emitting correct PDF widget annotation dictionaries for all four subtypes (text, checkbox, radio group, radio kid).
• OBJ_catalog — modified to emit an /AcroForm dictionary (with /Fields, /DR, /DA) when any form fields are present.
• pdf_create_form_xobject — helper creating Form XObject appearance streams without adding them to any page's content stream list.
• pdf_get_form_font — helper finding or creating a Helvetica Type1 font for use in form field rendering.
• Checkboxes render a path-based checkmark (✓); radio buttons render a filled circle approximated by four cubic Bézier segments (control-point distance 0.5523 ≈ 4/3·(√2−1)).
• Radio groups enforce single-selection: adding a second selected button to the same group returns an error.

tests/main.c

Added a new page 6 ("Form Page") demonstrating:

• A text field with a default value
• A checked checkbox and an unchecked checkbox
• A radio button group with three options (one pre-selected)

tests/tests.sh

Updated expected page count from 5 to 6.