Prepare v0.3.4 release

[rohoswagger]

Bumps to 0.3.4 and records the release. Cargo.lock is regenerated in the same commit because the release workflow runs cargo publish --locked.

What's in 0.3.4

#156 — ez split. Turn a multi-commit branch into a stack with one branch per commit, then ez submit for a PR each. The agent workflow it enables: commit freely on one branch, then ship the commits as a stack in one command.

It rewrites nothing — the commits already form a chain, so each layer is a branch ref pointed at a commit already in place plus a recorded parent. The branch you split keeps its name, tip, and any existing PR, becoming the top layer. Refuses before creating anything on a name collision or a merge commit in the range; no-ops on a single-commit branch; --dry-run previews.

#155 — ez move restructures the GitHub native stack it moves within. Previously it rebased locally, then called update_pr_base, which GitHub rejects for a PR inside a native stack — and reported "Moved" anyway, leaving GitHub's topology stale with nothing saying so. It now detects the native stack before any local mutation, skips the base edit that was never going to work, and dissolves/recreates the stack via repair_native_stack_exact. Receipts split local from remote, so "Moved" can no longer imply the remote followed.

Both commands are exercised by the release canary against real GitHub — ez split was added to scripts/github-canary.sh as the canary_lists_every_operational_top_level_command contract requires.

25 suites green, clippy clean.