release: prepare v0.4.0

[eareimu]

Release prep

• Bumps package dshell to v0.4.0.
• Depends on published crates.io h3x = "0.4.0" with the DShell-required rpc, serde, and webtransport features.
• Keeps DShell on the WebTransport/h3x API line that downstream gmutils and gateway consume.

Release gates

• Release surface: crates.io package dshell.
• Initial/incremental state: incremental (dshell already exists on crates.io at v0.3.0).
• Planned release tag: v0.4.0 after merge approval.
• Merge/tag/publish remain gated on final PR CI and explicit user approval.

Verification

• cargo metadata --no-deps --format-version 1
• cargo fmt --check
• cargo publish --dry-run --allow-dirty
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo build --no-default-features --features pam
• git diff --check