Fix concept-continuity slips in chapter prose

[mre]

What

A step-by-step continuity pass over the course, checking two things per chapter:

1. No concept is used before it's introduced (even briefly). Deliberate, clearly-labeled forward/back references are fine; the course teaches ownership as a spiral on purpose. This pass only touches cases where something is used cold, or a reference points the wrong way.
2. Every sentence is correctly placed (no dangling references to things that don't exist).

Chapters 00-02 were reviewed in detail by hand; 03-25 were inventoried (concepts introduced vs. used) and reconciled against the running ledger, then each flagged spot was verified before editing.

Changes (all prose / doc-comments, no behavior change)

Chapter	Issue	Fix
01 strings	# Your first function — but the reader implemented three functions in the integers chapter	Retitle to # A welcome message
04 functions	4_sum_to references a countdown exercise that exists nowhere in the course	Remove the dangling reference; introduce recursion fresh
10 tuples	Lists * deref among syntax "we haven't formally introduced yet" — but it was taught in the hashmaps chapter	Reword; turn it into a callback to hashmaps
14 structs	"the same ownership rules from the ownership chapter apply" — that chapter (memory & ownership) comes after this one	Point to the moves and borrowing chapters (which precede it)
19 password validator	Intro snippet uses ? in a String-returning function (won't compile; the chapter intro uses .unwrap() for the same code)	Use .unwrap()
19 password validator	References PasswordAdvisor, a type never defined anywhere	Drop the specific name
23 smart pointers	Says Box<dyn Error> is something "you'll meet in the ? operator chapter" — that chapter is earlier and defers Box<dyn Error> to the env-file parser chapter	Point to the env-file parser chapter, where it's actually introduced

Validation

• ./scripts/check-solutions.sh — 75/75 solutions compile + tests pass
• ./scripts/check-examples.sh — all chapters in their expected state

Notes / deliberately left alone

• The const fn in the structs chapter's scaffolding initially looked like an unexplained concept, but it's required to satisfy clippy's missing_const_for_fn under the examples check's -D warnings. Reverted to respect the repo's lint policy.
• Chapter 03's count_evens exercise uses &[i32] slices before the borrowing (05) and vectors (08) chapters. This is consistent with the documented "ownership spiral" design (introduce briefly at point of use, with a hint), so I left it; flagging here in case you'd rather restructure it.
• Chapter 24's quiz covers Rc/Arc/RefCell from the smart-pointers chapter, which is marked bonus. A learner who skips the bonus would meet those cold in the quiz. Not changed (design call), noted for awareness.