M7: inline symbols in text

[Haven-King]

What and why

Some type needs small pieces of art inside a line, such as icons in instructions, UI glyphs in documentation, or symbols in game and map legends. VectorCraft could only fake this with symbol-font glyphs and separately positioned objects.

• Model: TextRun.inline: Option<InlineArt> holds the symbol, scale and baseline_shift; the run is a single U+FFFC.
• Layout:
  • Placement:
    • The art is laid out as one glyph whose advance is the art width plus tracking.
    • It's scaled to scale × the font size and centred on the cap height.
    • Tall art makes its line taller.
  • Interactions:
    • It breaks like a word.
    • It follows bidi reordering in right-to-left text.
    • It counts as a character of its run's size for Character Alignment and Top-to-Top leading.
    • It never hangs as burasagari punctuation.
  • TextLayout.inlines records each placement.
• Drawing and export:
  • The canvas draws the symbol art inside the text's transparency group.
  • SVG keeps the text live and adds a <use> for each graphic.
  • PDF draws the art as vector paths.
  • A missing symbol keeps an empty one-em slot and is logged once.
• Editing:
  • Runs never merge into or split an inline run.
  • Backspace and Delete remove a graphic whole; copy keeps it.
  • Create Outlines turns it into a symbol instance.
  • Redefine Symbol resizes it.
• Translations: the menu item is translated in every catalog (cs, es, it, ja, pt-br, ru, zh-hans, zh-hant).
• Known gaps:
  • In vertical type the art lies on its side instead of standing upright.
  • symbol.list instance counts and Select All Instances ignore inline uses.
  • The menu item has no dialog for scale or shift.

How it was tested

• crates/text/src/tests_inline.rs:
  • Advance, placement, shift and tracking; a missing symbol; tall art.
  • Wrapping; type on a path and vertical type; snapping; editing; copy/paste; malformed runs.
  • Right-to-left placement, Character Alignment and Top-to-Top leading.
• crates/engine/src/tests_inline.rs: the command; junk parameters; Type tool with Backspace and undo; Redefine/Delete Symbol; save/open; Create Outlines.
• crates/engine/tests/inline_text.rs: canvas pixels and opacity; SVG and PDF export that re-import and render.
• command_sweep junk cases; native round trip. All symbol art in tests is drawn in code.

Checklist

• cargo xtask ci passes (fmt, clippy, tests, assets, brands, layers, wasm). All 7 steps pass on b843fe8, every test included.
• No panics in shipped code: no unwrap(), expect(), panic!, unreachable!, todo! or
  unimplemented! outside tests, and no unchecked indexing on data from files, parameters or
  messages. Errors are returned as Result and shown to the user
  (why and how). Clippy's no-panic lints pass on b843fe8.
• Clean-room: nothing copied from Adobe products or GPL/AGPL code (AGENTS.md).
• Every new asset has a row in ASSETS.md (no new assets).

[bflatastic]

Merged via #630, thanks @Haven-King! Inline symbols are a neat addition for instructions, legends and UI docs. Having them flow, wrap and reorder like a glyph, follow symbol edits, and export to SVG and PDF makes them first-class. I inserted a dot symbol into a line with text.insertInline and it rendered in place; undo removed it cleanly.