feat(core): wire IconvSetting to FilenameConverter at config build (#1911)

[oferchen]

Closes #1911.

Summary

Completes the local-copy bridge between the parsed IconvSetting on ClientConfig and the engine's LocalCopyOptions, mirroring the SSH / daemon bridge that PR #3458 wired through apply_common_server_flags. The audit at docs/audits/iconv-inert.md (PR #3526) labels #1911 as "Partial" because the local-copy path bypasses apply_common_server_flags and had no route by which a resolved FilenameConverter could reach the engine.

After this PR, every transport that hands a ClientConfig to the engine has an Option<FilenameConverter> available on LocalCopyOptions::iconv() for downstream wiring sites (#1912 sender emit, #1913 receiver ingest, #1914 filter-rule path matching) to consume. Wire encoding, application semantics, and producer-site changes remain out of scope and are tracked separately.

Changes

• engine::local_copy::LocalCopyOptions and LocalCopyOptionsBuilder gain an iconv: Option<FilenameConverter> field with a with_iconv setter and iconv() accessor on the options struct, plus an iconv() setter on the builder.
• core::client::run::LocalCopyOptionsBuilder::build (the internal wrapper around the engine builder) calls config.iconv().resolve_converter() via a new apply_iconv step and propagates the result onto LocalCopyOptions.
• Unit tests at crates/engine/src/local_copy/options/filters.rs cover default-none, attach, and clear semantics.
• Unit tests at crates/core/src/client/run/mod.rs cover the IconvSetting -> LocalCopyOptions.iconv mapping for Unspecified, Disabled, LocaleDefault, Explicit, and unsupported-charset variants.

Audit alignment

This is exactly the work called out as "Smallest-PR-first sequence" step 1 in docs/audits/iconv-inert.md section 4.2:

#1911 local-copy bridge - extend LocalCopyOptions (in crates/engine/src/local_copy/) with iconv: Option<FilenameConverter> and call config.iconv().resolve_converter() from crates/core/src/client/run/mod.rs::build_local_copy_options. Roughly +20 lines, no new types.

Net source change is +189 lines including tests and rustdoc. No new types.

Upstream references in the new code:

• flist.c::iconv_for_local
• options.c::recv_iconv_settings
• compat.c:716-718

Out of scope

This PR does not apply the converter on emit, ingest, or filter matching. Those producer wirings are #1912, #1913, and #1914 respectively. The hard-error-when-feature-off check is already in tree (#1915, PR #3458) so no iconv feature gate work is needed here.

Test plan

• cargo fmt --all -- --check clean.
• cargo nextest run -p engine --all-features -E 'test(iconv) + test(local_copy_options)' -> 56 tests passed (locally).
• cargo nextest run -p core --all-features -E 'test(iconv)' -> 24 tests passed (locally).
• cargo nextest run -p core --all-features -E 'test(iconv_wiring)' -> 5 new tests passed (locally).
• Full CI: fmt+clippy, nextest (stable), Windows (stable), macOS (stable), Linux musl (stable), interop.