docs(iconv): audit IconvSetting parse-path dead-end (#1909)

[oferchen]

Summary

Adds docs/audits/iconv-parse-deadend.md: a narrow trace of the
--iconv argument from clap entry through IconvSetting,
ClientConfigBuilder, ClientConfig, and out to its consumers,
identifying the residual dead-end after PR #3458.

The dead-end on the local-copy path is at
crates/core/src/client/run/mod.rs:275 (build_local_copy_options(&config, filter_program)):
the builder never reads config.iconv(), and LocalCopyOptions
exposes no setter for an Option<FilenameConverter>. Filter-rule
matching has the same gap at
crates/core/src/client/run/mod.rs:274 (compile_filter_program)
and the FilterChain::new(filter_set) call sites.

Complements the existing docs/audits/iconv-inert.md, which was
written before PR #3458 wired the SSH/daemon bridge and surveys the
full pipeline rather than the parse path.

Guarantees

• Docs only: single new file under docs/audits/, no code or other
  doc changes.
• Cites real symbols at real path:line for every step of the parse
  path and every consumer site.
• Matches the docs/audits/ style: tracking issue, scope, evidence
  with path:line citations, upstream cross-reference, summary.
• Follows conventional-prefix and hyphen-only prose rules.

Test plan

• File created at docs/audits/iconv-parse-deadend.md.
• Line count under 400 (348).
• No em-dashes in prose.
• Cited line numbers verified against the worktree.
• No source-code modifications.