Add fuzz target for ShortId parsing

[caarloshenriq]

Adds a libFuzzer target for payjoin::directory::ShortId, the 64-bit identifier used in Payjoin Directory URL path segments (/{id}).

What is covered

Three input paths are exercised:

• TryFrom<&[u8]>: raw 8-byte slices, as stored internally and compared against database keys. Any length other than 8 must be rejected; the target asserts this explicitly.
• FromStr: attacker-controlled bech32-without-checksum strings, the actual path exercised by payjoin-mailroom when routing incoming requests.
• Structured mutation (fuzz_near_valid): builds a valid encoding from the input seed, then corrupts it using the remaining bytes as a mutation opcode (truncation, single character substitution, case change, transposition, overlong segment, trailing garbage). This keeps the fuzzer near the parser's acceptance boundary rather than generating inputs that are rejected on the first invalid character.

All three paths assert that a successfully parsed ShortId round-trips through Display/FromStr. Sender and receiver both derive the mailbox URL from this string, so any asymmetry between encoding and decoding would split a session silently.

What is not covered

The network-facing OHTTP/bhttp handling in payjoin-mailroom is out of scope here. Covering those handlers would require adding tokio/axum/hyper to the fuzz crate. The next logical step is fuzzing the session routing and mailbox
matching logic using Arbitrary on payjoin protocol types, pending #1662 and the upstream rust-bitcoin Arbitrary release.

Fuzzing results

Run on x86_64 Linux, 6 forks (nproc - 2), 305 seconds:

Metric	Value
Total executions	~83.7M
Coverage counters reached	280 / 1345
Feature edges	875
Corpus size	151 inputs
Crashes / OOM / Timeouts	0 / 0 / 0

Coverage saturated at ~72s and held stable through the remaining ~230s, confirming the reachable space for this parser is fully explored. The target's value going forward is regression detection: any change to ShortId, bech32::nochecksum, or the Display/FromStr contract will surface here before reaching CI.

Addresses part of #1267.

Disclosure: co-authored by Claude

Pull Request Checklist

Please confirm the following before requesting review:

• I have disclosed my use of
  AI
  in the body of this PR.
• I have read CONTRIBUTING.md and rebased my branch to produce hygienic commits.

[coveralls]

Coverage Report for CI Build 32400373640

Coverage remained the same at 86.718%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	16458
Covered Lines:	14272
Line Coverage:	86.72%
Coverage Strength:	343.65 hits per line

---

💛 - Coveralls

[benalleng]

TACK 7200876

directory_short_id
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 1601642367
...
#4741050: cov: 280 ft: 874 corp: 156 exec/s: 139574 oom/timeout/crash: 0/0/0 time: 37s job: 7 dft_time: 0
INFO: fuzzed for 37 seconds, wrapping up soon

[erickcestari]

LGTM!

[benalleng]

ACK 8f4aad6