🔍 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/index.html

@@ -813,8 +813,13 @@ npm install ../cereale/cereale-0.3.0.tgz</code></pre>
       <div class="notes">
         <div class="note-item">
           <h3>Renaming is not backwards-compatible by itself</h3>
-          <p>Once a property carries <code class="inline-code">@JsonProperty</code>, its original name is
-            no longer accepted on input. Add <code class="inline-code">@JsonAlias</code> to keep older clients working.</p>
+          <p>Once a property carries <code class="inline-code">@JsonProperty</code>, its original name
+            no longer <em>maps</em> to it — but under the default
+            <code class="inline-code">unknownKeys: 'allow'</code> it is not rejected either. It is
+            copied onto the instance raw, skipping any
+            <code class="inline-code">@JsonType</code> or <code class="inline-code">@JsonDeserialize</code>
+            conversion declared for that field. Add <code class="inline-code">@JsonAlias</code> to keep
+            older clients working, or <code class="inline-code">unknownKeys: 'strip'</code> to drop them.</p>
         </div>
         <div class="note-item">
           <h3><code class="inline-code">abstract</code> and <code class="inline-code">accessor</code> fields cannot be decorated</h3>