types: add Struct iteration, and Extend for Struct, ListValue and FieldMask

[farmer-data]

Fixes #704.

Adds the collection items the issue lists, as one change:

• Struct: len, is_empty, contains_key, iter, IntoIterator for &Struct, &mut Struct and Struct, and Extend<(K, V)> with the Into<String> / Into<Value> bounds of from_fields. A key that is already present has its value replaced, as insert does. The docs state that iteration order is unspecified.
• ListValue: Extend<V: Into<Value>>, matching from_values.
• FieldMask: Extend<T: Into<String>>, appending without normalizing, as from_paths does.

Two additions beyond the plan I posted on the issue:

• Struct::iter_mut. IntoIterator for &mut Struct needs a mutable iterator anyway, and ListValue pairs the two the same way.
• Three public iterator types, StructIter, StructIterMut and StructIntoIter, re-exported at the crate root. Struct::fields is buffa::__private::HashMap, whose iterators are std's or hashbrown's depending on the std feature, so the IntoIterator impls need a type of their own to name. serde_json::Map takes the same approach. They forward fold and count to the map's iterators, implement ExactSizeIterator and FusedIterator (checked against the inner types in a test), and are #[must_use].

FromIterator keeps its existing item types ((String, Value) and Value). Widening it to match Extend could break inference in existing collect calls, so the crate docs describe the difference instead.

Tests don't depend on hash order. One generic test checks that C: Extend<T> + IntoIterator works for all three types, which is the issue's motivation.

Verification

• task lint, task doc, task check-nostd and task test-local (4211 passed, 0 failed), with the repository-pinned protoc 33.5. cargo +1.75 check -p buffa-types --all-features with an MSRV-resolved lockfile, as the msrv-check job does.
• Reviewed with the repo's rust-code-reviewer and rust-api-ergonomics-reviewer per CLAUDE.md: no Critical or High findings. Addressed the Mediums: the iterators forward fold/count, and the docs no longer overstate the FromIterator/Extend symmetry. Also addressed the Lows: a static check of the fused/exact-size claims, #[must_use], Extend doc wording, and HashMap::extend delegation.

[github-actions]

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[iainmcgin]

[claude code] Pushed one commit (3062fd2): two doc edits, with the code and tests unchanged.

• Extend for Struct: a.extend(b) moves the fields of b and does not carry over its unknown fields. The borrowed spelling a.extend(b.iter().map(|(k, v)| (k, v.clone()))) is given beside it, because a.extend(&b) does not compile where mask.extend(&other) does.
• The comment on the forwarded fold no longer lists collect, which drives next.

A follow-up for 0.10.0, outside this PR: FromIterator for Struct and ListValue will take the same item types as the Extend impls added here.