refactor(types): adopt derive_more and carve out logging-only bind

[mfw78]

What

Adopts derive_more for the audited newtype boilerplate (ContentDigest Display, FieldValue Display, ManifestSource From) and carves out a public bind_host_logging_via_wit_bindgen! entry point from the full wit-bindgen host binding macro.
Closes #48
Closes #103

Why

ContentDigest and FieldValue had hand-written Display impls and ManifestSource had hand-written From impls that derive_more replaces with equivalent, audited derive attributes, cutting boilerplate without changing observable output.
The logging-only macro exists because a domain world that remaps nexum:host/types onto its own SDK bindings makes the orphan rule block the whole macro, even though the logging arm only needs the freshly generated nexum::host::logging, so videre's domain SDK integration needs a base-block-free entry point.

Testing

Independent gate on nullislabs/nexum-runtime @ types/t7-derive-adoption (HEAD f0ba911 "refactor(runtime): derive the content-digest Display"), base origin/types/t6-typed-asserts. Fresh clone into /tmp/tmp.lVkmoFEtE9/repo, detached checkout, nothing reused from prior sessions. All commands run inside nix develop (rustc 1.94.0, nextest 0.9.127, sccache 0.16.0, mold 2.40.4).

Results (every step exit 0):

• cargo fmt --all -- --check: clean.
• Em-dash scan of added lines over base range: none (checked em dash and en dash).
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings: clean (1m59s).
• just build (engine + example + examples + fixtures, wasm32-wasip2): clean.
• CI=1 cargo nextest run --workspace --all-features --locked: 694/694 passed, 0 skipped, 0 failed.
• cargo test --doc --workspace --all-features --locked: all doctest targets ok, 0 failed.
• RUSTDOCFLAGS='-D warnings' cargo doc --workspace --no-deps --locked: clean (confirms the new intra-doc link to bind_host_logging_via_wit_bindgen! resolves).
• just test-e2e: 12/12 passed, 428 skipped by filter.

Identity/footers over origin/types/t6-typed-asserts..HEAD (5 commits: d50c792, d74f432, 3d247bf, 7df1fa8, f0ba911): author AND committer are mfw78 mfw78@nxm.rs on all five, no Co-Authored-By, no tool footers.

Diff review (7 files, +95/-37): ContentDigest hand-written Display replaced by derive_more::Display with a scheme-prefixed hex format, byte-identical output to the removed impl. FieldValue Display derived with default enum forwarding matching the removed match arms. ManifestSource From/From replaced by derive_more::From on exactly those two variants, Beside correctly left unmarked. New public bind_host_logging_via_wit_bindgen! is re-invoked from the logging arm of the full macro, so no duplicate item definitions on the full-bind path. New integration test crates/nexum-sdk/tests/wit_bindgen_logging.rs exercises the logging-only bind with no WitBindgenHost/nexum:host/types in scope and asserts the installed tracing facade reaches the bound log call. Cargo.lock carries the committed derive_more dep for nexum-sdk-test, consistent with --locked passing everywhere.

AI Assistance

Implementation by claude-fable-5, red-team review by claude-opus-5, PR authored by claude-sonnet-5.