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

Full rewrite. The previous README was explanatory but relied on the
car-rental analogy as its spine. This version restructures around the
sections the repo-wide overhaul uses everywhere:

  1. What does this program do? (plain English first; analogies only
     after the onchain mechanics, with each tradfi term defined
     briefly where it appears)
  2. Glossary (account, PDA, signer, CPI, Anchor constraint, bps,
     keeper, oracle, feed_id, exponent, etc.)
  3. Accounts and PDAs (state + vault tables; full field list of
     `Lease`; lifecycle diagram)
  4. Instruction lifecycle walkthrough (one subsection per ix, with
     signers / accounts / PDAs / token-flow diagrams / state changes
     / checks — in the order a user actually encounters them)
  5. Full-lifecycle worked examples (happy path, liquidation path,
     default-by-expiry, listed cancel — concrete numbers throughout)
  6. Safety and edge cases (full error-code table, guarded design
     choices, what the program does *not* guard)
  7. Running the tests (+ CI note confirming `anchor build` runs
     before `anchor test` in .github/workflows/anchor.yml, which
     covers the `include_bytes!` concern for §6)
  8. Extending the program (easy / moderate / harder tiers)

Reflects the code changes in this branch:
  * Fix 1 (close_vault helper extracted) in §Code layout
  * Fix 3 (LeasedMintEqualsCollateralMint) in §4.1 and §6.1
  * Fix 4 (feed_id pinning) in §2, §3, §4.6, §6
  * Fix 5 (last_rent_paid_ts on default path) in §4.7 and §5.3
  * Fix 6 (CI) confirmed in §7

1200 lines.

Author: Edward (Edwardbot)