test: /proc maps parser property tests + cargo-fuzz target; name PTRACE_GET_SYSCALL_INFO

[grloper]

• tests/maps_robustness.rs: dependency-free deterministic property tests (arbitrary bytes and mutated maps lines never panic; regions sorted; lookups never panic; well-formed example parses). Runs in normal cargo test.
• fuzz/: cargo-fuzz target maps_parse (separate workspace, no changes to main dependencies/Cargo.lock/MSRV).
• src/tracer.rs: magic 0x420e replaced by named const PTRACE_GET_SYSCALL_INFO (same value, as _ keeps the libc request type); unit test asserts equality with libc::PTRACE_GET_SYSCALL_INFO on gnu targets.

Ran locally (rustc 1.99): cargo build --release --locked, cargo test --locked (88 unit + all integration pass, 3 new robustness tests), cargo fmt --check, cargo clippy --all-targets --locked -D warnings, scripts/check_docs.py.
NOT run: the fuzz target itself (box has no C++ compiler/nightly+cargo-fuzz); MSRV 1.74 (left to CI).