feat(routing): add write path v1 CID-transform API

[lthibault]

Summary

• extend routing.capnp with write-path v1 operations:
  • mkdir(baseCid, path, parents) -> rootCid
  • writeFile(baseCid, path, data, createParents) -> rootCid
  • remove(baseCid, path, recursive) -> rootCid
  • publish(name, cid, expectedCurrent) -> publishedPath
• implement host-side handlers in crates/rpc/src/routing.rs
  • CID-transform workflow via isolated MFS workspace (no hidden mutable global root)
  • explicit IPNS compare-and-set conflict check for publish
  • path normalization / traversal rejection helpers
• wire new routing methods into shared Glia cap handlers (std/caps/src/lib.rs)
• document mutation semantics and conflict behavior (doc/capabilities.md)

Why

Stage 5 of #468: keep read-path on WASI filesystem I/O, while adding explicit, attenuable write operations that return new CIDs and avoid implicit mutable daemon state.

Validation

• cargo fmt --all
• cargo check -p ww --lib --bins
• cargo test -p ww test_publish_compare_and_set_semantics -- --nocapture
• cargo test -p ww test_normalize_rel_path_rejects_absolute_and_parent -- --nocapture

Refs #472
Refs #468