[M5] mxaccess-codec: F24 ASB Variant + AsbStatus + RuntimeValue codec

Ports `Variant` (cs:1170-1241), `AsbStatus` (cs:1109-1167), `RuntimeValue`
(cs:741-791), `AsbVariantFactory.From*` (cs:1310-1429), and
`MxAsbDataClient.DecodeVariant` (cs:713-825) into `mxaccess-codec::asb_variant`.

Three layers per `docs/ASB-Variant-Wire-Format.md`:
1. `AsbVariant` — raw 2/4/4/payload header + bytes; round-trips byte-identical.
2. `DecodedVariant` — typed view with one variant per proven ASB scalar / array
   (`Bool`, `Int32`, `Float`, `Double`, `String`, `DateTime`, `Duration` plus
   array forms). Type ids outside the proven matrix surface as
   `Unsupported { type_id, payload }` — same fallback as .NET's `_ => payload`.
3. `from_*` factories — mirror `AsbVariantFactory.FromX` exactly, setting
   `length` to `payload.len()` per `cs:1431-1438`.

`AsbStatus` and `RuntimeValue` round-trip the wire layout verbatim.
Status-element walking (marker bit 7 = implicit zero, etc., per
`docs/ASB-Variant-Wire-Format.md:180-205`) is deferred to a follow-up; the
codec exposes the raw status payload bytes for now, matching .NET's
`AsbStatus.Payload = byte[]` shape.

The lib.rs `AsbVariant` / `AsbStatus` / `RuntimeValue` stubs are replaced by
the real types via `pub use`. 25 new unit tests cover the proven matrix:
scalar + array round-trip, byte layout (2/4/4/payload), `Unsupported`
fallback for declared-but-unproven types, short-frame rejection,
malformed `string[]` partial-decode preservation matching .NET behavior.

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

design/followups.md

@@ -46,11 +46,15 @@ move to `## Resolved` with a date + commit hash.
 
 **Resolves when:** F19-F26 are all closed and the four DoD bullets above pass.
 
-**This-iteration execution slice (resolved in this commit).** F19 + F23 landed:
+**Cumulative execution log.** F19 + F23 landed in commit `ed17c07`; F24 landed in this commit:
+- F24: `mxaccess-codec::asb_variant` ports `Variant` + `AsbStatus` + `RuntimeValue` from `AsbContracts.cs:1109-1241,741-791` plus `MxAsbDataClient::DecodeVariant` + `AsbVariantFactory` from `cs:713-825,1310-1429`. Wire layout per `docs/ASB-Variant-Wire-Format.md`. `AsbVariant` is the raw 10-byte-header + payload form; `DecodedVariant` is the typed view; `from_*` factories mirror .NET's `From*`. 25 unit tests cover all proven scalar/array types' round-trip, byte layout (2/4/4/payload), `Unsupported` fallback for type ids outside the proven matrix, `AsbStatus` round-trip, `RuntimeValue` round-trip, malformed `string[]` partial-decode preservation, and short-frame rejection.
+
+**Earlier slices:**
+- F19 + F23 (commit `ed17c07`):
 - 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.
 
-F20-F22, F24-F26 remain open for parallel agent fan-out. F27 (constant-time DH) is filed as a separate follow-up below.
+F20, F21, F22, F25, F26 remain open for parallel agent fan-out. 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)