Add Facet support for indexmap::IndexSet

[TeamDman]

Fixes #2466

This adds Facet support for indexmap::IndexSet<T, S> under Facet's existing indexmap feature.

What changed:

• added a Facet impl for IndexSet<T, S> in the indexmap integration
• modeled IndexSet as a set
• added unit coverage for insertion, duplicate handling, iteration order, and deallocation behavior
• added format-suite coverage so JSON, TOML, YAML, MsgPack, Lua, and ASN.1 inherit round-trip testing

Before:

use facet::Facet;
use indexmap::IndexSet;

#[derive(Facet)]
struct Wrapper {
    values: IndexSet<String>,
}

This failed with:

error[E0277]: the trait bound `IndexSet<String>: Facet<'_>` is not satisfied

After:

• IndexSet derives/fields compile under the indexmap feature
• set semantics are preserved
• insertion-order iteration is preserved

Validation:

cargo test -p facet-core indexset --features indexmap
cargo test -p facet-json --test format_suite
cargo test -p facet-msgpack --test format_suite
cargo check -p facet-yaml --test format_suite -p facet-toml --test format_suite -p facet-lua --test format_suite -p facet-asn1 --test format_suite

Drafted with LLM assistance using Codex (GPT-5).

[codecov]

Codecov Report

❌ Patch coverage is 71.97802% with 51 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
facet-core/src/impls/crates/indexmap.rs	69.87%	50 Missing ⚠️
facet-format-suite/src/lib.rs	93.75%	1 Missing ⚠️

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