Refactor request completion and plot image ordering

[t-kalinowski]

Summary

Refactors the server/backend boundary for request completion and plot image ordering. The branch removes the worker-owned request_end sideband message, uses worker-declared client-waiting prompt events as the completion signal, and keeps response image identity/update grouping owned by the server.

Public-facing changes:

• No MCP tool schema changes.
• Tightens visible reply ordering for flushed text before plot image events.

Internal-only changes:

• Removes request_end from the worker-to-server sideband protocol.
• Keeps plot image sideband messages one-way; workers do not receive plot-image acknowledgements and must not gate stdout/stderr on sideband responses.
• Makes response image IDs server-owned while allowing workers to provide optional plot source identity.
• Prevents stale disclosed image-bundle anchors from replaying before fresh follow-up replies.
• Keeps explicit repeated Python plot() and show() calls image-producing while preserving quiet no-op requests.
• Surfaces strict IPC parse failures as disconnects instead of hidden backend-info or request-completion timeouts.
• Adds manual workflow_dispatch support to CI.
• Documents remaining server/backend boundary follow-up work.

Diff composition

Measured against main, this PR is 1896 insertions and 460 deletions across 32 files.

• runtime src/: +839/-315 (49.0% of churn)
• inline tests inside src/: +424/-54 (20.3% of churn)
• tests in tests/: +433/-22 (19.3% of churn)
• docs: +105/-27 (5.6% of churn)
• snapshots: +24/-0 (1.0% of churn)
• other: +71/-42 (4.8% of churn)

Largest files:

• src/ipc.rs: +514/-169
• src/worker_process.rs: +325/-111
• tests/python_plot_images.rs: +224/-0
• src/output_timeline.rs: +141/-39
• src/pending_output_tape.rs: +139/-14

Testing

• cargo +nightly fmt
• cargo check
• cargo build
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• codex review --base main