docs: generate crate READMEs from lib.rs with cargo-rdme, checked in CI

[no-materials]

What & why

The execution_graph README Quick Start didn't compile: the fn main() example had dropped ? on add_node and set_input_value (both return Result), producing two E0308s. A reader's first copy-paste from the docs would fail to build, while the lib.rs doctest stayed correct because only it was exercised by CI.

Rather than just patch the README, this fixes the root cause — the README was a hand-transcribed copy of the doctest, free to drift.

Approach

Adopt cargo-rdme, matching the convention used in other org repos (e.g. understory):

• src/lib.rs doc comments are the single source of truth. Each README body is generated from them, between <!-- cargo-rdme start/end --> markers.
• New cargo rdme CI job runs cargo rdme --workspace-project=<crate> --heading-base-level=0 --check for execution_tape and execution_graph, so a README can never drift from its docs again.
• The richer README content (Model, Execution behavior, Core Pieces, Design Docs, …) was migrated into the doc comments, so nothing is lost and the docs.rs pages now carry the full guide.
• The Quick Start examples are compiled and run by the existing cargo test --doc step (execution_tape's example was upgraded from no_run to executed).

Verification

• cargo rdme --check passes for both crates
• cargo test --doc --workspace passes
• cargo fmt --check, cargo clippy -- -D warnings, and cargo doc with --cfg docsrs -D warnings all clean

Updating a README

Edit the doc comment in src/lib.rs and run the same command without --check; a reminder comment is in each lib.rs and README.