[Rex16200513]

Export As can remember JPEG quality, but repeated deliveries still require setting the format and dimensions by hand. Add named presets for RGB PNG, JPEG, TIFF and PDF, so a configuration such as “Web JPEG, longest edge 1600 px” can be reused across documents.

The Export As sheet gains a Presets menu, Save Preset popover and Delete Preset submenu. Presets persist locally and store format, JPEG quality, background color, and either original dimensions or a longest-edge limit. A limit preserves proportions and never enlarges smaller images. Applying a preset updates the existing preview and leaves all controls editable. Names must be unique, nonblank and at most 80 characters.

This first version excludes CMYK profiles, 16-bit PNG, HDR, animation and document-specific channel mappings; Save Preset is disabled for those settings. Applying a screen preset explicitly selects RGB/8-bit settings. Presets do not change the document format.

Allow exports smaller than 1% so a large source can honor a small longest-edge limit. Remove the redundant format-change resize, which would otherwise recalculate a preset's scale from rounded width and distort the requested size on extremely narrow images.

Validation:

• Three Swift Testing tests passed in an isolated package using the actual ExportPreset.swift, DocumentLimits.swift and ExportPresetTests.swift: orientation/no-upscale/original-size rules, persistence/deletion, invalid/corrupt preferences.
• Swift syntax parsing and git diff whitespace checks passed.
• Full application build, integrated test target and visual UI verification remain unperformed: this machine has Command Line Tools but no full Xcode. Submitted as a draft for that reason.

Related: #268 (export templates and reusable export settings).