rewrite README as a concise pitch

Cut from 1215 lines to ~106. The README is now an intro plus a breadth
inventory of what the library can do, not a tutorial:

- one-paragraph pitch + the not-MISRA disclaimer
- "What it can do": one line per subsystem (allocators, containers, bignum,
  formatted I/O, pattern matching, parsers, system access, freestanding,
  lifetimes, safety, feature flags)
- "A taste": a Vec example shown with and without Scope (the lifetime split),
  plus a compile-verified Match/When/Otherwise pattern-matching showcase
- short Build / Feature flags / Docs sections, everything deeper linked out

Dropped the Perspective essay, Six Core Ideas, the container/parser/IO tours,
the freestanding deep-dive, and the table of contents.

Author: brightprogrammer