🔍 fix: correct the last three findings from the page review

Of 26 findings raised across five auditors, 23 were refuted on a second
pass. These three survived.

**A renamed property's old key is still writable.** The README, the landing
page and two doc comments all said that once a property carries
@JsonProperty its original name "is no longer accepted on input". It is no
longer *mapped* — but it is not rejected either. Unlike @JsonReadOnly,
whose JSON name goes into the blocked set, the old key falls through to the
unknown-key policy, and the default `allow` copies it onto the instance
untouched. Reproduced against dist:

  @JsonProperty('home_address') @JsonType(() => Addr) @ValidateNested()
  address!: Addr;

  toInstanceSync(Order, { address: { city: 'Paris' } })
    -> address is a plain object, instanceof Addr === false
    -> validateSync() returns []          <- nothing complains
    -> round-trips out as home_address    <- silently accepted

Blocking the old key would fix it, but would also swallow the
`unknownKeys: 'error'` report a strict caller gets today, which is arguably
the more useful signal. That is a judgement call the library has not made,
so this commit states the behaviour accurately everywhere it was stated
wrongly and pins it with five tests covering the default, `strip`, `error`
and the @JsonAlias fix — so it cannot drift either way while the question
is open.

**@IsNotEmpty and @IsEmpty are not complements.** `[]` and `{}` pass BOTH:
isNotEmpty checks only null/undefined/'' while isEmpty also treats empty
arrays and objects as empty. Listed one line apart as "must not be empty" /
"must be empty", they invited exactly the wrong inference.

**unknownKeys is deserialization-only**, in a group whose blurb says these
apply per call or via configure().

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

docs/page.js

@@ -132,8 +132,8 @@
       ["@IsDate()", 'must be a valid Date object'],
       ["@IsObject()", 'must be an object'],
       ["@IsDefined()", 'must not be null or undefined'],
-      ["@IsNotEmpty()", 'must not be empty'],
-      ["@IsEmpty()", 'must be empty']
+      ["@IsNotEmpty()", 'must not be null, undefined or an empty string — [] and {} pass'],
+      ["@IsEmpty()", 'must be null, undefined, an empty string, [] or {}']
     ]],
     ['Numbers', 'Constraints on number fields.', [
       ["@Min(n)", 'must be at least n'],
@@ -218,7 +218,7 @@
     ['Configuration', 'Per call, or once via configure().', [
       ["validate: boolean", 'validate while mapping — default true'],
       ["namingStrategy: strategy", 'identity (default), camelCase, PascalCase, snake_case, SCREAMING_SNAKE_CASE, kebab-case, or your own function'],
-      ["unknownKeys: policy", 'allow (default), strip, or error'],
+      ["unknownKeys: policy", 'allow (default), strip, or error — deserialization only'],
       ["maxDepth: number", 'nesting limit before a JsonMappingError — default 64'],
       ["configure(options)", 'sets the library-wide defaults'],
       ["getConfig()", 'reads the defaults currently in force'],