fix(build.mcpp): an unknown directive is not necessarily a typo

Adding `link-script` in protocol 3 proved the old wording wrong. It said:

    The program announced protocol 2, which this mcpp also speaks, so an
    unrecognized directive is a typo rather than newer syntax.

The premise does not hold. A build.mcpp's protocol number is substituted at
COMPILE time by whichever mcpp is running — it is not carried by the package —
so a package written against a newer mcpp arrives at an older one wearing the
OLDER engine's number. The two agreeing therefore says nothing about whether
the KEY is from the future, and this is exactly the case a board-support
package using `mcpp:link-script=` hits on an mcpp that predates it: told its
directive is misspelled, when the real answer is `mcpp self update`.

An old engine genuinely cannot tell the two apart. Naming both is the only
honest thing it can do, and the upgrade is the cheaper one to try first.

Author: speak-agent

src/build/directives.cppm

@@ -467,12 +467,27 @@ std::optional<std::string> protocol_error(const Directives& d) {
         std::string list;
         for (auto const& k : d.unknownKeys)
             list += (list.empty() ? "" : ", ") + ("mcpp:" + k);
+        // ⚠️ NOT "so it must be a typo".
+        //
+        // That is what this said, and adding `link-script` in protocol 3
+        // proved it wrong: a package written against a newer mcpp reaches an
+        // older one with the OLDER engine's protocol number stamped on it —
+        // the announcement is substituted at build.mcpp compile time by
+        // whichever engine is running, not carried by the package. So the two
+        // numbers agreeing says nothing about whether the KEY is from the
+        // future, and an old mcpp cannot tell the two cases apart. Naming both
+        // is the only honest thing it can do, and the upgrade is the cheaper
+        // one to try first.
         return std::format(
             "build.mcpp emitted directive(s) this mcpp does not know: {}.\n"
-            "       The program announced protocol {}, which this mcpp also "
-            "speaks, so an unrecognized directive is a typo rather than newer "
-            "syntax.",
-            list, d.protocol);
+            "       Either the package was written for a newer mcpp (try "
+            "`mcpp self update`),\n"
+            "       or the directive is misspelled. This mcpp speaks protocol "
+            "{}; the protocol number\n"
+            "       cannot distinguish the two, because it is stamped by "
+            "whichever mcpp compiled\n"
+            "       the program, not by the package.",
+            list, kProtocolVersion);
     }
     return std::nullopt;
 }