Better reflect::Struct field unpacking

[mkeeter]

(staged on #658)

Adds a Struct::field function which unpacks a strongly-typed field, then uses it in relevant places.

[labbott]

LGTM

[mkeeter]

Tested on niles to make sure I didn't mess anything up with the command changes

matt@niles ~ () $ env RUST_BACKTRACE=1 pfexec ./humility -t gimlet-d spd
humility: attached to 0483:3754:000D00344741500820383733 via ST-Link V3
ADDR MANUFACTURER              PART                 WEEK YEAR
   0 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   1 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   2 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   3 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   4 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   5 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   6 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   7 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   8 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
   9 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
  10 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
  11 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
  12 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
  13 Micron Technology         36ASF8G72PZ-3G2E1      34 2022
  14 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
  15 Micron Technology         36ASF8G72PZ-3G2F1       2 2023
matt@niles ~ () $ env RUST_BACKTRACE=1 pfexec ./humility -t gimlet-d net counters
humility: attached to 0483:3754:000D00344741500820383733 via ST-Link V3
 -------------------------------------------------------------------
 |  KSZ8463  |         Transmit         |         Receive          |
 |           |   UC   |   BC   |   MC   |   UC   |   BC   |   MC   |
 |-----------|--------|--------|--------|--------|--------|--------|
 | Port 1    |      0 |      0 |     33 |      0 |      0 |      0 |
 | Port 2    |      0 |      0 |     33 |      0 |      0 |      0 |
 | Port 3    |      0 |      0 |      0 |      0 |      0 |     66 |
 -------------------------------------------------------------------

 -------------------------------------------------------
 |     VSC8562     |     Transmit    |     Receive     |
 |                 |  Good  |   Bad  |  Good  |   Bad  |
 |-----------------------------------------------------|
 | Port 0 | MAC    |     33 |      0 |      0 |      0 |
 |        | Media  |      0 |      0 |     33 |      0 |
 | Port 1 | MAC    |     33 |      0 |      0 |      0 |
 |        | Media  |      0 |      0 |     33 |      0 |
 -------------------------------------------------------

            ┌──────────────────┐            ┌───────────────────┐
            │ KSZ8463          │            │ VSC85x2           │
            │                  │tx        rx│                   │tx
            │                33├───────────►│33               33├───────►
            │                  │1          0│                   │0
┌────┐    rx│                 0│◄───────────┤0                 0│◄───────
│    ├─────►│66                │rx        tx│                   │rx
│ SP │     3│                  │            │                   │
│    │◄─────┤0                 │tx        rx│                   │tx
└────┘    tx│                33├───────────►│33               33├───────►
            │                  │2          1│                   │1
            │                 0│◄───────────┤0                 0│◄───────
            │                  │rx        tx│                   │rx
            └──────────────────┘            └───────────────────┘
                                             MEDIA           MAC

matt@niles ~ () $ env RUST_BACKTRACE=1 pfexec ./humility -t gimlet-d net status
humility: attached to 0483:3754:000D00344741500820383733 via ST-Link V3
          ------------------              ---------------------
          |            UP  1 <----------> 0  UP          DOWN 0 <------>
  SP <--> 3  KSZ8463       |              |      VSC85x2      |
     RMII |            UP  2 <----------> 1  UP          DOWN 1 <------>
          ------------------  100BASE-FX  ---------------------  SGMII

(in fact, I had messed up one of the counter array sizes)

[Copilot]

Pull request overview

This PR improves reflected struct/tuple field access by adding Struct::field / Tuple::field helpers that deserialize strongly-typed members via Load, and then updates downstream call sites to use the new API.

Changes:

• Add Struct::field / Tuple::field typed accessors in humility-core::reflect, plus new Load impls (e.g., for Struct, Tuple, Base, ()).
• Refactor SPD parsing to use typed field extraction for nested SPD buffers.
• Refactor net command decoding (MAC address, link status, counters) to use typed field extraction.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
humility-spd/src/lib.rs	Simplifies nested SPD buffer unpacking using Struct::field.
humility-core/src/reflect.rs	Introduces typed field helpers and additional Load impls; tweaks a field lookup error string.
cmd/net/src/lib.rs	Replaces manual reflected decoding with Struct::field / Tuple::field in several command paths.

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