feat(course): SEO for the /learn/ pages

[gHashTag]

SEO for the course's static pages from #1428. Those pages were live but nothing linked to them.

• Linked.

  • The home page's static shell (scripts/boot-shell.ts, which crawlers see before React mounts) now links /learn/. The label is the course's own title from course.generated.ts.
  • Every course screen in the app links its static page: "Link to share, with a preview card: t27.ai/learn//".

• Snippets.

  • Titles carry the query: "Bits, trits and GF16: FPGA course, lesson 4 of 30" (RU: "...: курс по FPGA, урок 4 из 30"). This is new SAY_SEO_TITLE in the spec.
  • Descriptions are cut at a word before 155 characters; they were up to 285.

• Content. Each lesson page now also shows:

  • the widget's own description, read from the widget page's og:description, not copied;
  • the full source of the lesson's tutorial spec, for the 10 lessons that have one.

  bits-and-trits went from about 490 words to 930.

• Structured data.

  • Every page gets a BreadcrumbList.
  • The overview gets an ItemList of all 30 lessons.
  • The Course gets educationalLevel: Beginner.

• llms.txt. onboarding.t27 gets COURSE, COURSE_SOURCE and COURSE_NOTE. The generator refuses a course address that is off t27.ai.

Cards are unchanged: their HTML did not change, so nothing was redrawn.

Closes #1433

{
  "version": 1,
  "head_sha": "3632bcb8098daa719b3165aa2c5362e38dbe7463",
  "summary": "The course's static /learn/ pages are now linked from the home shell and the app, carry search-sized titles and descriptions, the lesson's spec source and widget description, breadcrumbs, and are named in llms.txt.",
  "changes": [
    "scripts/boot-shell.ts: the home shell links learn/ with the course title from course.generated.ts.",
    "src/pages/Course.tsx: CourseNotes links the static page of the course or lesson to share; new SAY_SHARE and SAY_SEO_TITLE in course.t27 and course.ru.json.",
    "scripts/course-pages.mjs: SEO title, descriptions cut before 155 characters, widget og:description and tutorial spec source on lesson pages, BreadcrumbList, ItemList and educationalLevel in JSON-LD; 62 pages regenerated.",
    "specs/catalog/onboarding.t27 and scripts/onboarding-from-spec.mjs: COURSE, COURSE_SOURCE and COURSE_NOTE, checked to be on t27.ai; llms.txt and agents.t27 regenerated."
  ],
  "tests": [
    {"command": "npm run check:course-pages", "result": "62 pages up to date; 6 tests pass, including breadcrumbs, ItemList and snippet lengths", "status": "passed", "evidence": "Local run on this branch."},
    {"command": "npm run check:course && npm run test:course", "result": "30 lessons, spec tests 6 and asserts 24 hold; 15 pass", "status": "passed", "evidence": "Local run on this branch."},
    {"command": "npm run check:onboarding && npm run test:onboarding-spec", "result": "up to date, spec tests 10 and asserts 55 hold; 4 pass", "status": "passed", "evidence": "Local run on this branch."},
    {"command": "vite build && npm run check:boot-shell", "result": "boot-shell contract ok, 12 links, dist/index.html carries it", "status": "passed", "evidence": "Local run on this branch."},
    {"command": "npm run typecheck:ratchet", "result": "179 errors across 26 files, baseline 179; no file gained", "status": "passed", "evidence": "Local run on this branch."}
  ],
  "limitations": [
    "eslint react-hooks/set-state-in-effect in Course.tsx is on main already; this change does not touch that effect.",
    "Search Console and Yandex Webmaster need the owner's sign-in; sitemaps are submitted there separately."
  ],
  "tags": ["Course", "Website", "SEO"],
  "blog": {
    "title": "Making the t27 course findable",
    "summary": "The course's static pages existed but nothing linked to them; now the home page and the app link them, and each lesson carries its own spec source.",
    "outline": [
      "Why a page reachable only from a sitemap ranks poorly, and how the home page's crawler-visible shell and the in-app share link fix that.",
      "Search-sized titles and descriptions generated from the course spec in two languages, with a test that holds them to the length a result shows.",
      "Giving thirty lesson pages enough of their own words: the tutorial spec's real source and the tool's own description, read rather than copied."
    ]
  }
}

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