engine: make the font-size commands \tiny…\Huge functional again

[tannevaled]

What was broken

A size clo (size10/size11/size12.clo) redefines \normalsize, \small, …, \Huge as \@setfontsize\<cmd>\<size>{<leading>}, and the engine's \@setfontsize is a no-op gobble. So after the class loaded, every size command was dead — a heading set with \Large, a caption with \small, a footnote with \footnotesize all rendered at the body size with the body leading. Measured before this change, every command sat at the base size with the base \baselineskip at 10pt, 11pt and 12pt.

The fix

Same route as #75 (which revived \normalsize's base size): drive the size through the font system, not by making \@setfontsize a live primitive. At \begin{document}, \gotex@sizeclosetup reads the (size, leading) the active clo baked into each command's body and redefines it as a \protected macro:

\gotexsize<permille>\relax\gotexleading<leading>sp\relax <clo display-skip/list tail>

• \gotexsize scales the class base font (as \large already did) to the clo's absolute point size.
• \gotexleading (new primitive) sets the matching \baselineskip, group-scoped via a new save-stack entry.
• The clo's own tail — \abovedisplayskip/\belowdisplayskip glue and \@listi params that \small/\footnotesize carry — is preserved (the dead gobble left it running).
• \protected keeps it robust in a moving context (a section title into the ToC, a caption into the LoF) — exactly the failure mode engine: honour a class's 11pt/12pt base size in the font system #75 avoided by refusing to make \@setfontsize functional.
• Reading off the active clo gives each class its own table: \Large in a 12pt document is the 12pt clo's 17pt on 22pt leading.
• \normalsize is deliberately not rewired (engine: honour a class's 11pt/12pt base size in the font system #75 owns its base size/leading; it is called on nearly every reset, so hardcoding a leading would fight \linespread/setspace).

Per-command proof (glyph size / \baselineskip), matches the clo byte-for-byte

base	tiny	scriptsize	footnotesize	small	large	Large	LARGE	huge	Huge
10pt	5/6	7/8	8/9.5	9/11	12/14	14/18	17/22	20/25	25/30
11pt	6/7	8/9.5	9/11	10/12	12/14	14/18	17/22	20/25	25/30
12pt	6/7	8/9.5	10/12	11/13.6	14/18	17/22	20/25	25/30	25/30

Layout impact — MIXED, not a clear win (please review before merging)

On the -layout harness (40 papers, seed 1, tectonic ground truth):

config	mean divergence	total page-count error
before (dead)	7.760	123
this PR (size + leading)	8.377	133
size-only variant	7.998	—

The aggregate is a net regression, driven almost entirely by one algorithm2e paper (2508.20008, whose \SetCommentSty is only partly supported): excluding that single outlier the mean improves 7.929 → 7.879. Heading-heavy papers gain (one 2608.07597: 13.76 → 6.29); caption/footnote/\small-heavy papers lose a little, because the engine already packs denser than tectonic and correct sizing amplifies that pre-existing line-break/page-build divergence. Counts: 3 improved (2 large), 7 slightly worsened, 18 unchanged.

This is a correctness fix (the commands were genuinely dead); the layout metric should reward it once dense-region breaking matches tectonic more closely. Opening for review, not merging.

Verification

• go test -race ./... green; conformance / faithfulness gates green; gofmt/go vet clean.
• New code (fontsizeclo.go, doLeading) at 100% statement coverage incl. error branches.
• engine: honour a class's 11pt/12pt base size in the font system #75's \normalsize/base-size behaviour unchanged.

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