docs: add Japanese translations for all chapters

[com-junkawasaki]

Hi Cliff-san,

This adds a complete Japanese reading path for Simple: the root chapter index, all 26 chapter READMEs (including 17a/17b), all 26 grammar documents, the Chapter 4 DCE appendix, the Chapter 21 encoding reference, and the Chapter 25 module, roadmap, and allocation-fixture notes (58 translations) for study.

Japanese documents live alongside the originals as *.ja.md. Chapter navigation, grammar links, and tables of contents point to the Japanese versions. Existing English documents only gain a language-switch link. Code samples, grammar productions, and diagram assets remain unchanged; figure captions and surrounding prose are translated. Translator notes identify existing mismatched grammar titles, dangling TOC entries, and conflicting source descriptions instead of silently changing their technical meaning.

Validation:

• All 58 source documents have a Japanese counterpart.
• All 348 fenced blocks and 86 Markdown image references match the originals exactly and in order.
• All relative file links and Markdown heading anchors in the Japanese documents resolve.
• Independently reviewed the translations against the English prose, including permissions, type lattices, compiler algorithms, and performance-comparison caveats.
• Existing English files are byte-identical after removing the added language links. Whitespace findings are confined to verbatim source code blocks; translated prose is clean.

Documentation-only change; compiler builds/tests were not run.

[com-junkawasaki]

Reviewed the Japanese documentation at 863a6ed against base 81b654d.

Independent structural verification: 58 translations, 116 Markdown-only changed files; all 348 fenced code blocks and 86 image references are identical and in source order. All 582 relative file references exist and all 160 heading anchors resolve using Markdown parsing and GitHub heading slug rules. English originals are unchanged except the language switch and its adjacent spacing. Existing trailing whitespace is inside copied source examples.

Focused prose review covered type-lattice meet, overflow/subword semantics, SSA/memory aliasing, cyclic types, instruction encoding, and the Chapter 25 design/benchmark boundary. The design proposals remain identified as proposals; historical allocation cohorts are not mislabeled as current native execution coverage. Source contradictions and dangling TOC sections are identified with translator notes rather than silently changing algorithms. No actionable issue found in this review.

Documentation-only: compiler builds were not run. The authenticated account has pull-only access to SeaOfNodes/Simple, and the PR requires an owner review; merging is not available to this account.