fix(pdf): clear a minimum font size per line block, and only where one bites

[andiwand]

🤖 Generated with Claude Code

Stacked on #752.

#752 lays a matrix-placed run out large and scales it down, because a producer
writing 1 Tf gets a font-size any minimum clamps. A minimum clamps a
uniform run just the same, so both paths need it — but doing it with a
constant 48 charges every block for it and lays a page of body text out at
400–900pt.

What this does

A line block is laid out at the 24px floor only when its own font-size
falls under it, and scales back by exactly that much: the uniform path
through a new z class, the matrix path through its own matrix. At or above
the floor the factor is 1 and the block is emitted byte for byte as before —
no transform, no z, nothing.

• text_local_scale = 48 → text_floor_pt = 18 (24px, the largest minimum a
  chrome user can set; an android webview defaults to 8px) plus
  local_scale(font_size_pt).
• run_geometry goes back to the natural matrix and size; each layer applies
  its own block's scale. Size is already in the visual and single-layer flow
  keys, so the factor is constant per block there; the selection layer groups
  by frame rather than size, so it carries the scale as block state and a run
  joining a block is measured in that block's space.
• .t no longer carries a transform.

The clamp reads the specified size, not the zoomed one — verified against
--blink-settings=minimumFontSize at zoom 1 / 0.5 / 0.25 — so body's fit
zoom does not move the floor.

Verification

Each page rendered with and without a browser minimum font size, diffed
against the unclamped render (the truth):

file	minimum	before	after
with_form.pdf	24px	8.8% of pixels wrong	0.000%, max delta 0
phd-agreement.pdf	24px	31.5%	0.000%, max 0
svg_background_with_page_rotation_issue402.pdf	8px	28.6%	0.000%, max 0
Casio_WVA-M650-7AJF.pdf	24px	20.0%	0.000%, max 0
mova-viax-500-user-manual-de.pdf	24px	4.1%	0.000%, max 0
geneve_1564.pdf	8px	1.0%	0.000%, max 0

With no minimum set, text lands in the same place at the same size; 0.2–0.9% of
pixels differ from glyph rasterization (the browser rasterizes e.g. 18pt scaled
by 0.667 rather than 12pt). A magnified crop confirms identical positions and
metrics.

Against the constant 48 on 978-3-030-65771-0.pdf: max font-size 60pt rather
than 2880pt, html 9.74 MB rather than 10.15 MB, 28 scale classes rather than
one transform on all ~25 000 line blocks. Load, scroll frame times and zoom
steps are unchanged across all three variants.

Note

Nearly every pdf's output changes, so test/data/reference-output needs
regenerating and the pointer advanced.