feat(fuzzer): add valid-address generator for G, M, and C kinds

[Legit003]

Implements random_valid_address(kind, rng) in src/generate.rs that produces correctly checksummed strkey for all three address types:

• G: version(0x30) + 32 random bytes + CRC-16 LE → 56 chars
• M: version(0x60) + random u64 muxed id (BE) + 32 random bytes + CRC-16 LE → 69 chars (exercises the full u64 decoder path)
• C: version(0x10) + 32 random bytes + CRC-16 LE → 56 chars

Every generated address is round-tripped through prism_core::address::parse immediately; a parse failure panics so a broken generator is caught at seed-generation time rather than producing silent bad corpus entries.

run_random in main.rs now emits one valid seed per three random strings (every 4th input), cycling G → M → C, so the fuzzer explores the boundary of validity rather than spending all budget on obvious garbage.

Also fixes two pre-existing broken test fixtures (53-char G addresses) in parse.rs and prism-core/src/address.rs — both now use the correct 56-char all-zero-key address GAAAAAA...AWHF.
closes #288

Summary by CodeRabbit

• Bug Fixes

  • Updated address validation tests with corrected valid and invalid examples.
  • Improved coverage for lowercase normalization and muxed-address parsing.

• Tests

  • Added generation and round-trip validation for correctly checksummed G, M, and C addresses.
  • Enhanced fuzz testing by mixing valid generated addresses with random inputs.
  • Added coverage for muxed identifiers, including boundary values.

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📝 Walkthrough

Walkthrough

The fuzzer gains a checksummed generator for G, M, and C addresses, injects valid seeds into random runs, and validates generated addresses through parsing. Parser test vectors are also updated.

Changes

Valid address fuzzing

Layer / File(s)	Summary
Checksummed address generator examples/rust-address-fuzzer/src/generate.rs	Generates G, M, and C strkeys with CRC-16 checksums, validates round-trip parsing, and tests lengths, prefixes, randomness, and muxed IDs.
Valid seeds in random fuzzing examples/rust-address-fuzzer/src/main.rs	Adds the generator module and periodically inserts valid addresses into random fuzzing inputs while retaining random strings.
Parser test vector updates examples/prism-core/src/address.rs, examples/rust-address-fuzzer/src/parse.rs	Replaces invalid, valid, and normalization address fixtures with updated strings.

Estimated code review effort: 4 (Complex) | ~45 minutes

Possibly related PRs

• Boxkit-Labs/stellar-address-kit#298: Modifies the same fuzzer entry points.
• Boxkit-Labs/stellar-address-kit#299: Updates overlapping parser test vectors.
• Boxkit-Labs/stellar-address-kit#300: Overlaps on parser fixtures and random-string range handling.

Suggested reviewers: codeze-us

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: adding a valid-address generator for G, M, and C kinds.
Linked Issues check	✅ Passed	The PR implements random_valid_address for G/M/C, uses random 64-bit M ids, and panics on parse validation failure.
Out of Scope Changes check	✅ Passed	The test-vector updates and fuzzer support changes align with the stated objectives and do not introduce unrelated scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (2)

examples/rust-address-fuzzer/src/main.rs (1)

79-93: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Fragile coupling between i % 4 and i % 12.

The kind-selection match i % 12 { 0 => G, 4 => M, _ => C } is only correct because the outer i % 4 == 0 guard guarantees i % 12 ∈ {0, 4, 8}. If either modulus is changed independently in the future (e.g. adjusting the valid-seed frequency), the _ => C arm would silently swallow unexpected residues instead of failing loudly, skewing the G/M/C distribution without any compiler or runtime signal.

Deriving the kind directly from the seed-slot index removes the implicit coupling:

♻️ Proposed refactor

-        let input = if i % 4 == 0 {
-            let kind = match i % 12 {
-                0 => AddressKind::G,
-                4 => AddressKind::M,
-                _ => AddressKind::C,
-            };
-            generate::random_valid_address(kind, rng)
-        } else {
-            random_string(rng)
-        };
+        let input = if i % 4 == 0 {
+            let kind = match (i / 4) % 3 {
+                0 => AddressKind::G,
+                1 => AddressKind::M,
+                _ => AddressKind::C,
+            };
+            generate::random_valid_address(kind, rng)
+        } else {
+            random_string(rng)
+        };

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@examples/rust-address-fuzzer/src/main.rs` around lines 79 - 93, Update the
valid-seed branch in the loop around random_valid_address so kind selection
derives from the seed-slot index rather than coupling i % 4 with i % 12. Use an
explicit exhaustive mapping for the intended G, M, and C sequence, and avoid a
catch-all arm that can silently accept unexpected residues; preserve
random_string for non-seed iterations.

examples/rust-address-fuzzer/src/generate.rs (1)

28-36: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Replace the hand-rolled CRC/Base32 helpers with crates. data-encoding::BASE32_NOPAD covers the Base32 path, and a CRC crate can replace the checksum helper; this keeps the fuzzer seed generator smaller and avoids maintaining protocol primitives locally.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@examples/rust-address-fuzzer/src/generate.rs` around lines 28 - 36, Replace
the local crc16 helper and the corresponding hand-rolled Base32 logic in the
seed generator with established crate implementations: use
data-encoding::BASE32_NOPAD for Base32 encoding and a suitable CRC crate for the
checksum, updating dependencies and call sites while preserving the current
output format and behavior.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@examples/rust-address-fuzzer/src/generate.rs`:
- Around line 28-36: Replace the local crc16 helper and the corresponding
hand-rolled Base32 logic in the seed generator with established crate
implementations: use data-encoding::BASE32_NOPAD for Base32 encoding and a
suitable CRC crate for the checksum, updating dependencies and call sites while
preserving the current output format and behavior.

In `@examples/rust-address-fuzzer/src/main.rs`:
- Around line 79-93: Update the valid-seed branch in the loop around
random_valid_address so kind selection derives from the seed-slot index rather
than coupling i % 4 with i % 12. Use an explicit exhaustive mapping for the
intended G, M, and C sequence, and avoid a catch-all arm that can silently
accept unexpected residues; preserve random_string for non-seed iterations.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: fa7cc5c6-1bfd-4cd8-8e01-62d2acea3b35

📥 Commits

Reviewing files that changed from the base of the PR and between d2898d4 and bf74f36.

📒 Files selected for processing (4)

• examples/prism-core/src/address.rs
• examples/rust-address-fuzzer/src/generate.rs
• examples/rust-address-fuzzer/src/main.rs
• examples/rust-address-fuzzer/src/parse.rs

[codeZe-us]

@Legit003 fix conflicts in your PR