🔍 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

README.md

@@ -507,8 +507,12 @@ it is one `Symbol.toStringTag` read per object.
 - **`validate()` on a plain object** returns no errors: rules live on the class, so validate
   the instance you get back from `toInstance`, not the raw payload.
 - **Renaming is not backwards-compatible by itself.** Once a property carries
-  `@JsonProperty`, its original name is no longer accepted on input — add `@JsonAlias` to
-  keep older clients working.
+  `@JsonProperty`, its original name no longer *maps* to it. Under the default
+  `unknownKeys: 'allow'` it is not rejected either: it is copied onto the instance raw,
+  skipping any `@JsonType` or `@JsonDeserialize` conversion declared for that field, so the
+  property ends up holding a plain object that `@ValidateNested` then finds nothing wrong
+  with. Add `@JsonAlias` to keep older clients working, or set `unknownKeys` to `strip` or
+  `error`.
 
 ## Contributing