Type: Proportional Metrics bring their kerning — apkn with palt, vkrn and vapk with vpal (#966)

The follow-up #966 left room for. With automatic kerning, horizontal type
with Proportional Metrics also applies `apkn`, and vertical type applies
`vkrn` and `vapk` along with `vpal`. Manual kerning turns them off, as it
turns off `kern`.

Vertical kerning used to cancel out: both top-to-bottom shapings had
harfrust's default `vkrn`. The baseline now has neither `vpal` nor the
kerning, so their difference carries both. A third shaping (`vpal` alone)
tells which advances `vpal` itself changed, and mojikumi steps aside only
for those. Vertical type without Proportional Metrics isn't kerned, as
before.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/mcp.md

@@ -2697,8 +2697,9 @@ OpenType `palt` feature; glyphs the font doesn't re-space keep their full width.
 the proportional heights of the font's `vpal` feature (the glyphs are shaped top to bottom for it) and move with them;
 a font without vertical metrics keeps its cells. Line-end Punctuation Half Width then takes nothing more off the
 punctuation `palt` or `vpal` re-spaced (the opening bracket at a line's start, consecutive punctuation, the closing
-mark at a line's end); punctuation left full width is trimmed as before. Off by default and saved only when on. One
-undo step.
+mark at a line's end); punctuation left full width is trimmed as before. With automatic kerning the font's kerning
+for the re-spaced glyphs comes too: `apkn` with `palt`, and `vkrn` and `vapk` with `vpal` (vertical type is otherwise
+not kerned). Off by default and saved only when on. One undo step.
 
 ```json
 {"name":"run_command","arguments":{"command":"text.setFormat","params":{"proportionalMetrics":true}}}