fix(query): percent-decode query parameters, and reach repeated keys

Context::query returned the raw slice, so `?q=hello%20world` read back as
"hello%20world" and `?q=a+b` as "a+b". It also split the query string after
lossy-decoding it to text, so an escaped separator inside a value (`%26`, `%3D`)
could still be mistaken for a real one. Repeated keys were unreachable: the first
match returned and there was no way to ask for the rest.

Splitting now happens over the raw bytes and each half is decoded afterwards with
the percent/plus decoder the urlencoded body parser already uses. Context::query_all
returns every value given for a key, in order.

Covered by query_decode_wbtest.mbt (escapes, plus-as-space, escaped separators,
repeated keys, absent keys); mutation-checked in both directions.

Also starts tracking the library's pkg.generated.mbti, which this repo had been
missing while its siblings carry theirs — it is the interface consumers read, and a
diff in it is the signal that a change is externally visible. The examples
regenerate their own, so those stay out of git.

Signed-off-by: Leo Cheng (heke1228) <chengkelfan@qq.com>

Author: Lfan-ke

.gitignore

@@ -2,3 +2,6 @@
 !.git*
 _build/
 target/
+
+# example packages regenerate these; only the library interface is tracked
+examples/*/pkg.generated.mbti