docs(clob): rewrite README to the repo-wide quality bar

The previous README was a ~78-line stub. This version describes the
program as it actually exists today, including the matching engine
landed in ea96084.

Structure matches the rest of the overhaul:

  1. What does this program do? (onchain mechanics first, with
     tradfi terms — limit order, order book, maker/taker, price-time
     priority — briefly explained in plain English before they get
     used)
  2. Glossary (account, PDA, CPI, bps, bid/ask, tick size,
     unsettled balance, price improvement, remaining_accounts, etc.)
  3. Accounts and PDAs (four program PDAs + three vaults; full
     field lists; note the vaults are not PDAs, they are regular
     token accounts whose authority is the Market PDA)
  4. Instruction lifecycle walkthrough (six instructions, in the
     order a user encounters them; per-ix signers / accounts / PDAs
     created / token-flow diagrams / state changes / checks)
  5. The matching engine — step by step (the critical section:
     how place_order uses remaining_accounts; the plan/apply/clean/
     fee/rest five-step structure; fee math; price improvement;
     worked fill walkthroughs including a multi-maker sweep)
  6. Full-lifecycle worked examples (clean match + settle,
     partial fill + remainder, cancel + settle round trip)
  7. Safety and edge cases (full error table; guarded design
     choices; what the example does NOT do)
  8. Running the tests (all 23 tests listed and categorised; CI
     note confirming anchor build runs before anchor test)
  9. Extending the program (easy / moderate / harder)

1433 lines. No code changes.

Author: Edward (Edwardbot)