docs(iconv): design minimal FilenameConverter trait (#1910)

[oferchen]

Summary

Audit confirms a FilenameConverter already exists at
crates/protocol/src/iconv/converter.rs as a concrete struct backed by
encoding_rs. Task #1910 therefore reduces to a confirm-and-document
exercise, with three minor refinement recommendations rather than
mandates:

• Rename the byte-oriented Cow methods (local_to_remote /
  remote_to_local) to align with the tracker description
  (convert_send / convert_recv) and promote &Path / Cow<Path>
  to the public boundary.
• Surface the unsupported-charset case as an rsync_error!(1, ...)
  rather than the current silent tracing::warn! -> None fallback.
• Align with Switch Windows cross-compilation to native cargo builds #1915 on the no-feature stub (hard error at config build
  when --iconv is supplied without the iconv Cargo feature).

The audit also documents the three application sites that need a
converter (sender flist emit, receiver flist ingest, filter-rule path
matching), justifies the struct-not-trait recommendation, defends
encoding_rs over system iconv on cross-platform / unsafe-policy
grounds, and confirms crates/protocol/src/iconv/ as the correct
home crate.

This is the design follow-on to PR #3514
(docs/audits/iconv-parse-deadend.md) and unblocks the wiring tasks
#1911-#1919.

Test plan

• CI checks pass

Related

• Sibling audit: docs/audits/iconv-parse-deadend.md (PR docs(iconv): audit IconvSetting parse-path dead-end (#1909) #3514)
• Older partial audit: docs/audits/iconv-inert.md
• Pipeline coverage: docs/audits/iconv-pipeline.md
• Cross-cutting design: docs/audits/iconv-feature-design.md
• Tracker: Add CI orchestrator validation to docs guardrails #1910
• Unblocks: Propagate fallback signal exit codes #1911, Cache proxy authorization headers #1912, Ensure fallback binary lookup mirrors Windows PATHEXT #1913, Ensure nextest failures fail the Linux test workflow #1914, Switch Windows cross-compilation to native cargo builds #1915, Accelerate rolling checksum with SIMD fast paths #1916, Handle files-from relative entries with explicit source directory #1917, Accelerate rolling checksum dispatch with AVX2 fast path #1919
• Closes-document: Add reusable fallback override interpreter #1918 (precondition)