test(LibBytecode): pin the documented sourceCount(hex"00") == 0 invariant explicitly

[thedavidmeister]

Gap

LibBytecode.sourceCount NatSpec documents a specific equivalence: "0x and 0x00 are equivalent, both having 0 sources." The "" (empty) case is pinned by testSourceCount0, but the single-byte hex"00" case has no explicit, named assertion in test/src/lib/bytecode/LibBytecode.sourceCount.t.sol.

It is covered indirectly:

• testSourceCount1 fuzzes bytecode.length > 0 and asserts sourceCount(bytecode) == uint8(bytecode[0]), so hex"00" is within its input domain and is asserted to return 0;
• checkNoOOBPointers tests exercise hex"00" as well.

So this is LOW / test-hardening, not an open bug — the behavior is fuzz-covered; only a dedicated assertion for the documented invariant is missing.

Proposed fix

Add one explicit unit test to LibBytecodeSourceCountTest:

/// The NatSpec documents 0x and 0x00 as equivalent (both 0 sources). `testSourceCount0`
/// pins the empty case; this pins the single 0x00 byte explicitly.
function testSourceCountSingleZeroByte() external pure {
    assertEq(LibBytecode.sourceCount(hex"00"), 0);
}

Provenance

Surfaced from the internal audit run audit/2026-03-01-01 (pass 2, finding A01-2), which was not carried into that run's triage.md (the triage indexed P2-A01-1 HIGH — bytecodeToSources coverage — but dropped this LOW). Filed so the finding is tracked before the resolved audit-run dirs are removed in #114.