PDF stage 3.6: design — Type3 glyphs + non-embedded fonts

Seed the stage-3.6 branch. Type3 char procs -> SVG via a minimal path->SVG
capability pulled forward from stage 4; non-embedded standard-14 substitution
+ AFM widths (closes stage 2's deferred item). Stacked on 3.5.
Implementation follows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014hm5SrdJvGNJNEHxpxR1dz

Author: andiwand

docs/design/pdf/stage-3.6-type3-nonembedded.md

@@ -0,0 +1,84 @@
+# PDF stage 3.6 — Type3 glyphs + non-embedded fonts
+
+Design for the final stage-3 sub-stage. Status: **design draft** (no
+implementation yet — this PR seeds the branch). Roadmap entry lives in
+[`src/odr/internal/pdf/AGENTS.md`](../../../src/odr/internal/pdf/AGENTS.md).
+
+Two loose ends that don't fit the "read a font program, wrap to OTF" pipeline:
+fonts whose glyphs are **drawing procedures** (Type3) and fonts with **no
+embedded program at all** (the standard 14 + common substitutes). Closes stage
+2's deferred AFM-widths item.
+
+## Part A — Type3 fonts
+
+Type3 glyphs are mini content streams, not outlines, so they cannot go through
+`@font-face`. They need path → SVG, which otherwise belongs to stage 4; a
+**minimal path → SVG capability is pulled forward here** (decision 2026-06-19)
+rather than waiting on the full graphics stage.
+
+### What gets read
+
+A Type3 font dictionary carries:
+- **`/CharProcs`** — glyph name → content stream (drawing procedure).
+- **`/Encoding`** — code → glyph name (Differences).
+- **`/FontMatrix`** — glyph space → text space (Type3 fonts set their own, not
+  the implicit 1/1000).
+- **`/FontBBox`**, **`/Resources`** — resources referenced by the procs.
+
+### Rendering
+
+Each char proc is already tokenized by the operator parser. The proc begins with
+`d0` (colored) or `d1` (uncolored — glyph takes the fill color, bbox follows).
+Run it through a small **path → SVG** emitter covering the path-construction +
+painting subset (`m`/`l`/`c`/`v`/`y`/`re`/`h`, `f`/`F`/`f*`/`S`/`s`/`B`/`b`,
+`W`/`W*` clip, `cm`, color ops) — the minimal slice of stage 4. Each glyph
+becomes an SVG `<symbol>`/`<g>` in glyph space; the HTML layer instantiates it
+per show at the text transform (CTM × Tm × FontMatrix, font size folded in),
+sized by `/FontMatrix`.
+
+Type3 has no glyph program → no PUA re-encode, no reverse map. Unicode for
+selection still comes from the stage-1 chain (`/ToUnicode` / `/Encoding`); the
+dual-layer model holds (SVG glyph layer + transparent Unicode layer).
+
+This path → SVG emitter is written to be **reused by stage 4** for page-level
+vector content.
+
+## Part B — non-embedded fonts
+
+A font with no `/FontFile*`: substitute and metric-match rather than render true
+glyphs.
+
+- **Substitution** — map the standard 14 (Helvetica/Times/Courier families +
+  Symbol/ZapfDingbats) and common names to CSS `font-family` fallback stacks
+  (serif/sans/mono by flags + name heuristics; bold/italic from the descriptor
+  `/Flags` and the name). No `@font-face`.
+- **Metrics** — drive placement from the PDF `/Widths` when present; for the
+  standard 14 (which usually ship no `/Widths`) use the **AFM advance-width
+  tables**, closing stage 2's deferred item. AFM widths become a generated data
+  table (`tools/pdf/generate_*`), like the encoding / AGL tables in
+  `pdf_encoding_data`.
+- Glyph shapes are the browser's fallback font — display fidelity is bounded
+  here by design (no program to embed); selection/search are exact.
+
+## Module touchpoints
+
+- `internal/svg/` (or `internal/font/type3_*`) — the minimal path → SVG emitter
+  (new, shared with stage 4).
+- `html/pdf_file.cpp` — Type3 SVG glyph emission; non-embedded substitution +
+  font-family stacks.
+- `pdf_encoding_data` (or sibling) — generated AFM width tables for the standard 14.
+- `pdf_document_element` — Type3 `/CharProcs`/`/FontMatrix`/`/Resources` on
+  `Font`; substitution facts for the non-embedded path.
+
+## Scope / non-goals
+
+- Only the path-painting operator subset needed by real Type3 procs; full stage-4
+  graphics (shadings, images, patterns) stays in stage 4.
+- No font synthesis for non-embedded fonts — substitution + metrics only.
+
+## Tests
+
+Type3: an inline content-stream font (a `d1` proc drawing a rectangle) asserting
+the emitted SVG path and its placement transform. Non-embedded: standard-14 AFM
+width lookup through `advance_width`, and the substitution family-stack mapping
+for a few representative names/flag combinations.