doc: add end-to-end tutorial

doc/tutorial.md walks from probe detection to JEDEC ID over EXTEST and
forward-references the BSCAN proxy path. Includes:
- prerequisites and build/launch
- chain scan, FPGA identification, registry lookup
- IR/DR primitive sanity check via the IDCODE register
- SPI JEDEC ID over EXTEST (with the speed caveat)
- recipe to add a new FPGA target
- troubleshooting cheat sheet

README and CLAUDE.md updated to point at it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -35,6 +35,7 @@ modules/             — Viveris's library (LGPL, unchanged) —
 └── natsort/         Natural pin-name sorting
 bsdl_files/          BSDL files for target FPGAs
 scripts/             Example scripts
+doc/                 Tutorial and longer-form docs (doc/tutorial.md is the end-to-end walkthrough)
 libs/libftd2xx/      Vendored FTDI SDK
 ```