descriptor: resolve each stored field once in DynamicMessage::for_each_set

[serhiy-bzhezytskyy]

Item 1 of #432.

What changed

• for_each_set applies the presence rule to the value it is already holding instead of calling has(fd), which looked the same field number up in the field map a second time (buffa-descriptor/src/reflect/dynamic.rs:1650-1663 on 053fcf3; in debug builds has also re-resolved the descriptor for its membership assert).
• The rule lives in one private helper, is_present(fd, value), used by both has and for_each_set; the comment that explained it moved with it.
• Visited fields, their order and the ValueRefs handed to the callback are unchanged: has looked up fd.number(), and field_or_extension(number) returns the descriptor with exactly that number for declared fields and extensions alike, so it found the same value the loop holds.

Why

Per stored field the loop did a BTreeMap step, a descriptor lookup by number, and then a second BTreeMap lookup inside has for a value it already had. Measured on the benchmark datasets, for_each_set alone over pre-decoded messages, release build, 2,000 iterations × 3 runs, same binary on main and on this branch:

dataset	set fields	main, ns/field	this branch, ns/field
AnalyticsEvent (50 messages)	250	10.3 – 11.3	6.2 – 6.5
GoogleMessage1	10	16.4 – 17.5	8.9 – 10.5

The existing reflect/*/dynamic_read_all criterion benches include the decode and move by less than their noise, so the isolated timing is the number. The probe is not part of this change.

Item 2 of the issue — a borrowing set_fields() iterator beside the trait method — is an API addition and is left for a separate change once its shape is decided.

Tests

• dynamic_message_for_each_set_agrees_with_has: an implicit-presence scalar at its default (skipped) and non-default, an optional field at its default (visited), and a message field; asserts the visited set equals the set of fields for which has() is true, and equals [14, 16, 17].
• cargo test -p buffa-descriptor --features reflect — 199 pass
• cargo test -p buffa-test — 564 pass

Checks

• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all --check
• changelog fragment under .changes/unreleased/

Written with Claude Code.

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[serhiy-bzhezytskyy]

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