[octalide]

Closes #169

Plan: router.bad_parameter, the error every built-in decoder returns, now answers the neutral invalid parameter, since the same decoders serve path captures and form.typed query values. The router dispatch test and the query typed__route_decoders test pin the text on both paths. The Go bench comparator answers the same text so the two servers stay byte-for-byte comparable.

Decisions the issue did not settle:

• The three router.compile sites keep invalid route parameter. They reject a route's parameter declaration at compile time, never a client value, so the text names its source correctly (the issue's "names the source" option).
• No CHANGELOG edit. The family rule is that changelogs are written at release from the merged commits, so the release entry should note the changed public text.

Ran: mach test . --filter dispatch__typed_params and --filter typed__route_decoders, mach fmt --check on root and demo.