[austin-barrington]

QueryParam::encode returns a Cow so it can borrow when a value doesn't need escaping, but EncodedParams then called .to_vec() on every value regardless. That meant a large text param got copied even though we're consuming the QueryParams and already own the String.

Changes:

• Params are consumed through a new private into_encoded(). Text that needs no escaping and Raw values are moved through as-is, no copy. Anything that does need escaping gets encoded like before.
• Arrays are written straight into one String. Before, each element got its own String, then they were joined, then wrapped with format!.
• The name/value buffers are sized up front.

Numbers from a quick timing loop (release, 20 runs each):

• 100k element Array(UInt64): ~63ms → ~60ms
• 8MB String: ~124ms → ~118ms
• single small scalar: no change

Most of the time is the engine parsing the value, so it's not a huge win, but it drops a full copy of every value on the way in. The encoded output is identical, the existing encoding tests all pass unchanged, and there's a new test that checks plain text and raw values really come back as the same allocation.

Note

Remove per-element string allocation in query param encoding

Rewrites query param value encoding in query_param.rs to write directly into a single output buffer instead of allocating an intermediate String for each nested element and joining them.

• QueryParam.encode_array and the new write_array helper build array literals (brackets, separators, elements) in place, reserving capacity from the element count.
• write_nested and write_quoted replace the allocating encode_nested and quote_nested_string helpers; escaping rules for backslashes, tabs, newlines, and apostrophes are unchanged.
• New QueryParam.into_encoded consumes the value so unescaped text and raw owned strings move into the encoded result without copying. EncodedParams.encode preallocates its buffer vectors from the collection length.
• Adds a test asserting that owned unescaped text and raw values keep their original allocation, and that tab-containing text is escaped.
• Behavioral Change: emitted parameter strings, lengths, and pointers exposed to the C FFI call are unchanged; only allocation behavior differs.

^{Macroscope summarized 6d75a1d.}