[M5] mxaccess-asb-nettcp: F21 [MC-NBFX] binary XML token codec

Ports the proven subset of `[MC-NBFX]` to `mxaccess-asb-nettcp::nbfx`.

Token model: Element { prefix, name } / EndElement / Attribute /
DefaultNamespace / NamespaceDeclaration / Text. Element + attribute
names can be inline UTF-8, an `[MC-NBFS]` static-dictionary id (via
F22's `lookup_static`), or a per-session `DynamicDictionary` id.

Text records covered: Empty (0xA8), Zero (0x80), One (0x82), Bool
(0x84/0x86 + 0xB4), Int8 (0x88), Int16 (0x8A), Int32 (0x8C), Int64
(0x8E), Chars (0x98/0x9A/0x9C — width variant chosen automatically by
payload length), DictionaryText (0xAA — both static and dynamic refs).

`*WithEndElement` collapse is automatic: a `Text → EndElement` pair
encodes as the `+1` record byte (e.g. `EmptyTextWithEndElement = 0xA9`,
`TrueTextWithEndElement = 0x87`). The decoder splits the implicit
EndElement back out so consumers see the same token stream regardless
of which wire form was used.

Element variants covered: ShortElement (0x40), Element (0x41 with
prefix string), ShortDictionaryElement (0x42), DictionaryElement
(0x43). Prefix-letter family (0x44-0x77) deferred — emit the long
form for now.

Attribute variants covered: ShortAttribute (0x04), Attribute (0x05),
ShortDictionaryAttribute (0x06), DictionaryAttribute (0x07), plus
xmlns variants (0x08/0x09).

15 new unit tests cover the dynamic dictionary, every supported
element/attribute/xmlns/text record form (including round-trip),
explicit byte pinning for the collapse behavior, Chars width-variant
selection, unknown-record rejection, and truncated-payload rejection.

Records left for follow-up: Decimal, UniqueId, TimeSpan, Float/Double
text, DateTime text, Bytes8/16/32, QNameDictionary, the 0x0C-0x25
prefix-dict-attribute / 0x26-0x3F prefix-attribute / 0x44-0x77
prefix-element families. None of these are on the proven ASB path.

With F21 landed, the M5 framing + encoder layer (streams A+B+C+D and
the F24 codec) is complete. F25 (mxaccess-asb IASBIDataV2 client) and
F26 (Session over AsbTransport) remain.

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

design/followups.md

@@ -46,7 +46,11 @@ move to `## Resolved` with a date + commit hash.
 
 **Resolves when:** F19-F26 are all closed and the four DoD bullets above pass.
 
-**Cumulative execution log.** F19 + F23 (`ed17c07`); F24 (`7611d9e`); F20 (`9dfd193`); F22 landed in this commit:
+**Cumulative execution log.** F19 + F23 (`ed17c07`); F24 (`7611d9e`); F20 (`9dfd193`); F22 (`43c10a1`); F21 landed in this commit:
+- F21: `mxaccess-asb-nettcp::nbfx` ports the `[MC-NBFX]` `.NET Binary XML Format` token codec — the proven subset for ASB. Token model: `Element { prefix, name }` / `EndElement` / `Attribute { prefix, name, value }` / `DefaultNamespace` / `NamespaceDeclaration` / `Text`. Name forms: inline UTF-8, `[MC-NBFS]` static-dictionary id, per-session `DynamicDictionary` id. Text forms: Empty, Zero, One, Bool, Int8/16/32/64, Chars (Chars8/16/32 width variants chosen automatically), and `DictionaryText` static/dynamic refs. The `*WithEndElement` text variants are collapsed automatically: `Text → EndElement` pairs encode as the `+1` record byte (e.g. `EmptyTextWithEndElement = 0xA9`); decoder splits them back out so consumers see the same token stream. 15 unit tests cover the dynamic-dictionary semantics, all element/attribute/xmlns/dict-text record forms, the collapse behavior with explicit byte pinning (`0x87` TrueTextWithEndElement, `0xA9` EmptyTextWithEndElement), Chars width-variant selection (Chars8 / Chars16 / Chars32 by length), unknown-record rejection, and truncated payloads. Records left for follow-up: Decimal, UniqueId, TimeSpan, Float/Double text, DateTime text, Bytes8/16/32, QNameDictionary, the `0x0C-0x25`/`0x26-0x3F` prefix-attribute and `0x44-0x77` prefix-element families.
+
+**Earlier slices:**
+- F22 (commit `43c10a1`):
 - F22: `mxaccess-asb-nettcp::nbfs` ports `[MC-NBFS]` §2.2 static dictionary table — the curated subset (~80 entries) covering SOAP 1.2 envelope, WS-Addressing 1.0, xsi/xsd primitives, common XML element/attribute names. `lookup_static(id)` and `position_of_static(value)` plus a `OnceLock`-cached reverse map. Lookups against unmapped IDs return `None` so the F21 NBFX decoder surfaces a clear error rather than silently corrupting. Extending the table is a one-line append in numerical order; existing tests assert monotonic IDs to catch transposition.
 
 **Earlier slices:**
@@ -62,7 +66,7 @@ move to `## Resolved` with a date + commit hash.
 - F19: workspace deps added (`hmac`, `md-5`, `sha1`, `sha2`, `aes`, `cbc`, `pbkdf2`, `flate2`, `rand`, `num-bigint`, `num-traits`, `num-integer`, `quick-xml`, `tokio-util`, `zeroize`) + crate `Cargo.toml` propagation.
 - F23: `mxaccess-asb-nettcp::auth` ports `AsbSystemAuthenticator` (167 LoC .NET → ~480 LoC Rust + tests). 13 tests cover decimal-prime parsing, .NET `BigInteger` byte-order round-trip (sign-byte append/strip + zero), base64 against RFC 4648 §10 vectors, public-key range, private-key sizing, peer-to-peer DH shared-secret agreement, signed-validator message-number monotonicity, AES-CBC PKCS7 padding, unknown hash algorithm fallback (no MAC unless `force_hmac=true`), Apollo `:V2` lifetime-suffix dispatch, PBKDF2-SHA1 self-consistency snapshot.
 
-F21, F25, F26 remain open for parallel agent fan-out. F22's static dictionary subset is intentionally curated; expand entries as wire captures show new IDs. F27 (constant-time DH) is filed as a separate follow-up below.
+F25 (`mxaccess-asb` IASBIDataV2 client) and F26 (`mxaccess::Session` over `AsbTransport`) remain open. With F19-F24 landed, the M5 framing/encoder layer (streams A+B+C+D and the codec stream) is complete; F25 composes them into the `IASBIDataV2` wire client. F22's static dictionary subset is intentionally curated; expand entries as wire captures show new IDs. F27 (constant-time DH) is filed as a separate follow-up below.
 
 ### F27 — Constant-time DH `mod_exp` (swap `num-bigint` → `crypto-bigint::BoxedUint`)
 **Severity:** P2 (security regression vs the long-term Rust target — but at parity with the .NET reference today, so not a release-blocker)