Add comprehensive fuzz targets for all workspace crates

[jrey8343]

Summary

• Add 7 new fuzz targets covering the entire rust-url workspace (url, idna, percent-encoding, form_urlencoded, data-url)
• Include roundtrip invariant checks, differential testing, and mutation testing
• Add seed corpus, fuzzing dictionary, and CIFuzz workflow for continuous fuzzing on PRs

Fuzz Targets

Target	Crate(s)	Strategy
fuzz_url_parse_roundtrip	url	Parse → serialize → re-parse roundtrip + component consistency
fuzz_url_differential	url	Relative URL resolution, join/make_relative roundtrip
fuzz_url_setters	url	Mutation via all setters, post-mutation validity invariant
fuzz_idna	idna	domain_to_ascii ↔ domain_to_unicode roundtrip, Punycode roundtrip
fuzz_data_url	data-url	DataUrl::process + decode, forgiving base64
fuzz_form_urlencoded	form_urlencoded	Parse → serialize → re-parse roundtrip
fuzz_percent_encoding	percent-encoding	Encode → decode roundtrip across multiple AsciiSets

Motivation

This is part of an effort to integrate rust-url into OSS-Fuzz for continuous fuzzing. URL parsing is a classic fuzzing target — it processes untrusted input, implements a complex spec (WHATWG URL Standard), and the workspace includes several sub-crates (IDNA, Punycode, percent-encoding, form-urlencoded, data-url) that each independently parse untrusted data.

The existing url/fuzz/ targets only cover the url crate itself. These new workspace-level targets extend coverage to all sub-crates with invariant-checking strategies that are most likely to surface real bugs.

Test plan

• All targets compile with cargo check in fuzz/
• Targets run successfully with cargo fuzz run <target> -- -max_total_time=60
• CIFuzz workflow triggers on PRs

[jrey8343]

Hi — apologies for pushing this PR without coordinating more closely first. I should have discussed with you before adding fuzzing infrastructure.

That said, the fuzz targets did uncover a couple of bugs (#1101, #1102) which I've submitted a fix for in #1103. Hopefully that demonstrates some value.

If you'd prefer a lighter-weight approach, I'd be happy to set up ClusterFuzzLite instead — it runs in your GitHub Actions CI so you'd have full control. Just let me know what works best for the project and I'm happy to help however I can.

[Manishearth]

Thank you for doing all this! I don't have time to review this soon but I appreciate the effort! I'm in favor of merging these.