Print: JIS B4/B5/B6 and Japanese paper sizes

Japanese Excel's B4/B5/B6 are the JIS sizes (257×364, 182×257 and 128×182 mm),
but Page Setup's `b4`/`b5` were the ISO ones (250×353, 176×250): a Japanese B5
worksheet printed 8 mm narrow. `known_paper` now reads `B4`/`B5`/`B6` as JIS and
keeps the ISO sizes reachable as `B4 (ISO)`/`B5 (ISO)`/`B6 (ISO)`. This also
fixes the XLSX round-trip, whose `paperSize` ids 12/13 are the JIS sizes.

Adds the Japanese papers Page Setup lacked: Japanese Postcard (はがき,
100×148 mm), Japanese Double Postcard (往復はがき, 148×200 mm), Japanese
Envelope #3/#4 (長形3号/4号, 120×235 and 90×225 mm), Japanese Envelope Kaku
#2/#3 (角形2号/3号, 240×332 and 216×277 mm), A6, and aliases such as
`postcard`, `chou #3`, `kaku #2`; the xlsx paper table gains ids 42 (ISO B4) and
43/69–74/88 so these names survive save and reopen (the ISO B5/B6 names have no
ECMA-376 paper id and stay engine-side).

The pageSetup/pageLayout.size parameter docs list the names, the Page Layout
Size menu lists them, and tests pin the point sizes, the PDF MediaBox for A4
(unchanged), B5, postcard, double postcard and kaku #2, landscape and
fit-to-page, the unknown-name Letter fallback, and the XLSX round-trip.

Docs: docs/target-app-parity.md print row + revision history, and ROADMAP.md
progress log, dated 2026-10-21.

ROADMAP.md

@@ -2,7 +2,7 @@
 
 **Stage: alpha** · next: beta, ~25 points of "ready for real work" and ~160–240 h away
 
-> **Last reviewed:** 2026-10-11 · **Last updated:** 2026-10-12 · **Change:** minor (rows auto-fit oversized fonts on open and whole-row/column formatting) · **Target:** Microsoft Excel (Microsoft 365)
+> **Last reviewed:** 2026-10-11 · **Last updated:** 2026-10-21 · **Change:** minor (Page Setup paper sizes: JIS B4/B5/B6, the Japanese postcards and envelopes, and A6) · **Target:** Microsoft Excel (Microsoft 365)
 
 GridCraft aims at full Microsoft Excel parity, and at being better in places: faster, open (XLSX
 is the native format), fully scriptable by agents (MCP, control channel, CLI), and on the web.
@@ -132,6 +132,7 @@ Ranked; estimates in Opus 5.5 agent-hours. Detail and the full beta-gate list in
 
 | Date | What landed |
 |---|---|
+| 2026-10-21 | Page Setup paper sizes match Japanese Excel: `B4`/`B5`/`B6` are the JIS sizes (ISO B4/B5/B6 stay reachable by name), so a Japanese B5 worksheet prints at 182×257 mm; Japanese Postcard (100×148 mm), Double Postcard (148×200 mm), Envelope #3/#4 (120×235/90×225 mm), Kaku #2/#3 (240×332/216×277 mm) and A6 are added, both sizes reach the PDF page (`file.exportPdf`/`file.print` MediaBox) and the size survives an XLSX round-trip; the Page Layout Size menu lists them. `B4`/`B5` used to be the ISO sizes, which is changed here |
 | 2026-10-12 | Rows auto-fit oversized fonts: opening a file and whole-row/column formatting grow non-custom rows to their tallest font (#833) |
 | 2026-10-12 | Number formats: parsing a format code is linear in its length (long runs of `[`, `,` or `/` no longer take quadratic time), and an unclosed `[` is a literal when splitting sections as well as when tokenizing (#810) |
 | 2026-10-12 | Number formats: a fraction denominator with 65,536 zeros no longer crashes; format codes are capped at 255 characters like Excel (longer codes display as General, `TEXT` gives `#VALUE!`) (#807) |
@@ -193,6 +194,7 @@ Ranked; estimates in Opus 5.5 agent-hours. Detail and the full beta-gate list in
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-21 | minor | Page Setup paper sizes: JIS B4/B5/B6 (ISO by name), Japanese postcards and envelopes, A6; PDF page size follows, XLSX round-trips |
 | 2026-10-12 | minor | Rows auto-fit oversized fonts on open and whole-row/column formatting (#833); progress log row |
 | 2026-10-12 | minor | Progress log: multi-area row/column delete (#758) |
 | 2026-10-12 | trivial | Progress log: XLSX chart sheets kept on save (#727) |