Reject encoded separators in non-greedy path parameters

[puffball1567]

Summary

• reject percent-encoded forward slashes and backslashes in non-greedy path parameters before URL decoding
• preserve ordinary and double-encoded values, as well as the documented multi-segment behavior of greedy parameters
• add matrix coverage for route position, separator position and casing, malformed encodings, HTTP methods, and greedy parameters

Tests

• nim c -r tests/mock/tmock_mocking/tmock_route.nim
• nim c -r -d:usestd tests/mock/tmock_mocking/tmock_route.nim
• nimble tests

Fixes #296

[ringabout]

Reviewed PR #310 against devel. The guard is applied before storing non-greedy captures, handles case-insensitive %2F/%5C, and leaves greedy parameters and double-encoded values unchanged. Focused tests pass on the default and -d:usestd backends. The regex-route behavior is pre-existing and can be followed up separately. LGTM.