PDF stage 2.1: text transforms & the placed-text emission

[andiwand]

🤖 Generated with Claude Code

First slice of stage 2 (text positioning & metrics). It applies the full text
transform chain and introduces a renderer-agnostic placed-text emission — the
core emits placed text + geometry + Unicode, and the renderer owns the
per-run-vs-per-glyph mapping decision. Glyph advances are deliberately deferred to
stage 2.2 (which branches off this PR).

What's in here

• pdf_geometry.hpp — a 2-D affine Matrix (compose, point-apply,
  translation/scaling factories), PDF row-vector convention.
• GraphicsState — the CTM now concatenates on cm (it was being
  overwritten — a latent bug); Tm/Tlm are tracked as Matrix values with
  BT/Td/TD/T* and the line-move half of '/"; text_placement_matrix()
  folds in horizontal scaling and rise, keeping font size separate.
• pdf_page_text (extract_text) — emits one TextElement per show
  operation (Tj/TJ/'/"), positioned by the placement matrix and carrying
  font/size/spacing/codes/Unicode. Lenient font lookup (unknown ref → warn).
• html/pdf_file.cpp — maps each TextElement to an absolutely-positioned
  span with a CSS transform (PDF user space → page box in CSS px, glyphs kept
  upright); routed through Logger; the debug std::cout and the "hi" marker
  are gone.

Deliberately out of scope (→ stage 2.2)

Glyph advances (/Widths, /W//DW) and the application of char/word spacing
and TJ numeric adjustments — so consecutive shows on a line without an explicit
move still overlap, and TJ renders its strings concatenated at one origin.
Precise baseline placement (needs font ascent metrics) is likewise deferred — the
baseline currently sits at the span's box top.

Tests

• test/src/internal/pdf/pdf_geometry.cpp — Matrix compose/apply, ordered
  composition.
• test/src/internal/pdf/pdf_page_text.cpp — Td/Tm/cm/Tz/Ts, TJ
  concatenation, T*/'/" line moves; inline content streams.
• Full suite green (461 tests), including the end-to-end HtmlOutputTests.

The roadmap in src/odr/internal/pdf/AGENTS.md is updated with the full stage-2
sub-stage split (2.1–2.6).