README: make the first code block runnable, move install up, dedupe Quickstart/Try-It-Now

[higagan]

From the 4-persona UX review. Hit by all four personas — it's on the most-copied part of the page.

Problem

Three compounding issues:

1. The first code block can't run. Quickstart (lines 24–30) calls an undefined smtp.send(...) and never actually calls send_email, so the first thing a reader pastes raises NameError. The runnable block is the second one (Try It Now, line 40).
2. pip install modelfuzz is at line 219 — ~180 lines below the first code, past the whole demo and scanner sections.
3. Quickstart and Try It Now are two near-identical send_email decorator blocks. Multiple reviewers thought the second was a dupe/typo.

Fix

• Lead with the working Try-It-Now block; delete the duplicate Quickstart block (or keep Quickstart but make it runnable — replace smtp.send(...) with print(...) or # your mailer here (SMTP, SES, …)).
• Put a one-line pip install modelfuzz directly above the first code block.
• Collapse the bare-vs-called explanation (currently stated twice, lines ~32 and ~34) into a single line after the working example.

Done when

• The first code block on the page runs as-is and prints its documented output
• pip install appears immediately above it
• No duplicated decorator block