fix(client): forward --iconv to remote rsync server args

[oferchen]

Summary

Adds --iconv to the long-form server-args forwarded by RemoteInvocationBuilder::append_long_form_args. Without this, the spawned rsync --server ran with no iconv context and emitted raw bytes on the wire - gap 1 of the standalone:iconv-local-ssh known failure.

The mapping mirrors upstream options.c:2716-2723 exactly:

IconvSetting variant	Forwarded flag
Unspecified	(none)
Disabled	(none - upstream nulls iconv_opt at options.c:2052-2054)
LocaleDefault	--iconv=.
Explicit { local, remote: Some(r) }	--iconv={r} (post-comma half only)
Explicit { local, remote: None }	--iconv={local} (no comma => forward whole)

Five new unit tests cover each row.

Upstream Reference

// options.c:2716-2723
if (iconv_opt) {
    char *set = strchr(iconv_opt, ',');
    if (set) set++;          // points to char *after* the comma -> "REMOTE"
    else set = iconv_opt;
    args[ac++] = safe_arg("--iconv", set);
}

Scope: this does NOT close the KF

This is a focused fix for gap 1 only. The standalone:iconv-local-ssh known failure stays open because:

• Gap 2: UTF-8 wire mismatch on the file-list side is still unresolved
• Iconv audit findings 1, 2, 3, and 5 remain open

The KF entry in tools/ci/known_failures.conf is intentionally unchanged.

Test plan

• CI: fmt + clippy
• CI: nextest (stable) - 5 new iconv_* tests in crates/core/src/client/remote/invocation/tests.rs
• CI: Windows / macOS / Linux musl matrices
• Confirm standalone:iconv-local-ssh remains in KNOWN_FAILURES (intentional)