fix(build): decode literal text in UDP payload probes

[kydallaboutlearning]

SNMP agents never answered RustScan's port-161 probe because the nmap-payloads decoder in build.rs kept only ASCII hex digits: the literal public community string collapsed to the single byte 0xbc, producing a 28-byte probe instead of 33 (and NetBIOS/LDAP/SSDP probes with literal text were mangled the same way).

This rewrites the decoder to Nmap's C-style escape format: \xNN and single-char escapes decode, every other character contributes its literal bytes, and multi-line quoted segments concatenate with no separators (honoring \" escapes). Also passes the block text to the decoder from its opening quote (previously the first quote was pre-stripped, breaking segment pairing), and strips CR line endings so CRLF checkouts parse identically to LF ones.

Verified end-to-end through the generated payload map:

• SNMP public-walk probe for port 161 is now the exact 33-byte BER packet
• SSDP probe for port 1900 decodes escapes + literal text across both segments

Gate (matches just test): cargo test all green, cargo clippy -- --deny warnings + cargo clippy --tests -- --deny warnings clean, cargo fmt --check clean, cargo doc clean.

Closes #933