Hardware track: course map, prerequisites, capstone receipts and a shared lesson-quality checklist #1481

Parent epic: #1476

With 4 shipped courses and 10 proposed, the course page needs structure for hardware engineers.

Proposal

1. Track page /#/course/track/hardware: a graph of courses with prerequisites (data in specs/course/courses.t27: new COURSE_PREREQS, COURSE_TRACK). Suggested order:
   t27-basics -> course (FPGA) -> {clocks-and-cdc, verify-hardware, arithmetic-circuits} -> {buses-and-peripherals, open-toolchain, ai-numbers} -> {ai-accelerator, ternary-computer, dsp-and-radio, power-and-test} -> fpga-to-silicon.
2. Capstone receipts: every course ends with a capstone whose result is a build receipt (hashes of spec, netlist, bitstream, board run) the reader can share as a black-and-white card.
3. Level and time per course (beginner / intermediate / advanced, hours) shown on the card.
4. Lesson-quality checklist in specs/course_recipe/course-27.t27 (one home for the rule): spec runs in browser or is a named cast; numbers have a DATA_SOURCES entry; board named; no duplicate of a lesson in another course (a check over all LESSON_SPECS/LESSON_IDS).
5. Duplicate-lesson check in CI: the same spec or lesson id used in two courses must be an explicit cross-link, not a second lesson.
6. Interview mode (optional): each module ends with 3 questions a hardware interviewer would ask, answers checked by spec tests.

Acceptance

• courses.t27 carries prerequisites and track; course page renders the graph.
• Duplicate-lesson check runs in CI and passes on all shipped courses.
• Recipe spec carries the quality checklist; AGENTS.md points to it (no copy).