[M5] mxaccess-asb-nettcp: F20 [MS-NMF] .NET Message Framing record codec

Implements the 13 record types from `[MS-NMF]` §2.2 (Version, Mode, Via,
KnownEncoding, ExtensibleEncoding, Unsized/SizedEnvelope, End, Fault,
UpgradeRequest/Response, PreambleAck, PreambleEnd) over a `net.tcp` channel.

Includes the `Multibyte Int31` length codec (LEB128-style 7-bit groups
over a 31-bit unsigned range, max 5 bytes; rejects negative input and
overflow), plus an `encode_preamble` helper that emits the canonical ASB
connect record sequence (`Version 1.0 → Duplex → Via $uri →
BinaryWithDictionary → PreambleEnd`).

Pure codec — no I/O. Encoders write into a `Vec<u8>`; decoders parse
from a `&[u8]` slice and return the consumed-byte count alongside the
record. Higher-level connect/request/response orchestration stays in the
M5 ASB client (`mxaccess-asb`, F25).

24 new unit tests cover round-trip for every record type, multibyte-int31
boundary cases (0, 1, 127, 128, 16383, 16384, 200, i32::MAX), preamble
emission against the canonical ASB sequence, byte-layout pinning for
Version/Mode/KnownEncoding, and rejection of unknown record/mode/encoding
bytes plus truncated sized-envelope frames.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rust/crates/mxaccess-asb-nettcp/src/lib.rs

@@ -20,3 +20,7 @@
 #![forbid(unsafe_code)]
 
 pub mod auth;
+pub mod nmf;
+
+pub use auth::AuthError;
+pub use nmf::{NmfEncoding, NmfError, NmfMode, NmfRecord, NmfRecordType};