Use bgpkit-parser v0.20.0 upstream text dump module

[digizeph]

Summary

• Bump bgpkit-parser 0.19.0 → 0.20.0 and replace Monocle's local 783-line Cisco sh ip bgp text dump implementation (src/lens/parse/text_dump.rs) with a re-export of the upstream bgpkit_parser::parser::text_dump module. All monocle::lens::parse::text_dump::* call sites keep working unchanged; the parser now carries the text dump test coverage (17 tests).
• Adapt MRT export call sites (monocle parse, monocle search) to bgpkit-parser v0.20.0's fallible encoding API (upstream issue #313): export_bytes() returns Result<Bytes, EncodingError> and MrtRibEncoder::process_elem returns Result; encoding failures are reported instead of silently truncating.

Verification

• cargo fmt clean; cargo clippy --all-features -- -D warnings clean (only pre-existing proc-macro-error2 future-incompat note)
• cargo test --all-features: 304 tests passed (279 lib + 15 parse + 1 + 9), 0 failed, 29 ignored
• End-to-end smoke: monocle parse on a synthetic PCH-style sh ip bgp dump produces correct BgpElem JSON (1.33.0.0/24, AS path [4657, 9318], router ID → peer_ip, local AS → peer_asn) via the re-export chain

Closes the follow-up from the v0.20.0 parser release: single text dump implementation, no fork.