Type: vertical type takes upright glyphs' cells from the font's vertical metrics

[imachus]

What and why

Upright glyphs in vertical type advanced one em (or their horizontal
advance, if wider) and turned about a fixed point 0.38 em above the
baseline. Fonts that carry vertical metrics now place them as they ask:

• FontFace::vertical_glyph: a glyph's advance down the column (vmtx)
  and the height of its vertical origin (VORG, else the glyph's top
  plus its top side bearing, as in TrueType fonts); None without
  vhea/vmtx or with values outside a few ems.
  FontFace::ideographic_centre: the em box centre from an ideograph's
  cell, else 0.38.
• The layout advances an upright glyph by its vertical advance and hangs
  it from its vertical origin; tate-chu-yoko blocks and point type's
  anchor use the font's em box centre. Fonts without vertical metrics
  are laid out exactly as before.
• Hiragino's tables give the values the layout assumed (one em, origin
  0.88 em), so Japanese vertical type in it renders pixel for pixel the
  same; this is the base for proportional vertical metrics (vpal) and
  for fonts whose em box sits elsewhere.
• test_fonts::vertical_font: Source Sans 3 with vhea/vmtx (and
  optionally VORG) built in code, one glyph 1.4 em tall hanging from
  1.1 em; tested with and without VORG.

First of a few small steps toward Japanese composition (the ROADMAP's "CJK composition for vertical type"), following the W3C Requirements for Japanese Text Layout and the OpenType spec (vhea, vmtx, VORG).

How it was tested

• cargo test for text, engine, render, pdf, svg and tools passes (2,135), with the new test in tests_vertical.rs (the test font is built in code; nothing is committed).
• Rendered vertical point and area type in Hiragino Mincho and Hiragino Sans before and after: identical images. Hiragino's vertical_glyph reads (1000, 880) for 国、、, ー and ゃ.
• cargo fmt --check, cargo clippy --all-targets -D warnings, cargo xtask brands, layers pass; the web build compiles.

Checklist

• cargo xtask ci passes (fmt, clippy, tests, assets, brands, layers, wasm).
• No panics in shipped code; table values outside a few ems are ignored.
• Clean-room: behaviour from the OpenType spec and JLREQ.
• No new assets.

🤖 Generated with Claude Code

[bflatastic]

Thank you @imachus! This is merged. Upright glyphs in vertical type now take their cell from the font's vhea/vmtx/VORG. Building the test font in code from Source Sans 3 is a neat way to pin it. While reviewing it I fixed one doc comment in #383: vertical_glyph returns two values, not three.

Checked live: MS Gothic vertical columns render correctly, with the vert forms ﹁﹂ and 。 placed at the top right of their cells.