Generalize `GraphCase` with an `external_static_default_edges` dimension
modelling each importing module choosing one of four local-name variants for
the default export of a given external module. Two modules picking different
variants for the same external triggers rolldown's nondeterministic
default-binding merge from #9754.

Plumbing in `output_fuzz_common`:

- Mirror the dimension through `acyclic_graph_case_strategy`,
  `case_from_seed`, `build_case_from_masks`, the `xs=` case-spec key, and
  the REPL URL.
- `render_graph_modules` emits `import dn_<ext>_<variant> from "external-<ext>"`
  and a `globalThis.__use_…` assignment so the import survives tree-shaking;
  CJS modules fall back to a deterministic `require` (the bug is ESM-only).
- `run_deterministic_check` now bundles each case four times against a
  baseline; the rolldown#9754 flip is probabilistic per build, so two runs
  were not enough to catch it at default `PROPTEST_CASES`.

Plumbing in `acyclic_output_fuzz`:

- `format_failure_message` gains an "Input External Static Default Imports"
  section that lists each importer with its chosen local name.
- `dynamic_hub_case_strategy` sets the new field to `Vec::new()` to keep its
  hub-cycle focus.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

The fixture generator works off the shared `GraphCase` machinery, so park it
next to `output_fuzz_common::generate_fixture_from_*` instead of the
acyclic-only test crate. Both fuzz crates can now point at the same
reproduce command.

- Move `generate_fixture.rs` and its `[[bin]]` entry into
  `output_fuzz_common`; drop them from `acyclic_output_fuzz`.
- Add a shared `output_fuzz_common::fixture_command(&case)` helper and use
  it from `acyclic_output_fuzz::format_failure_message`.
- Extend `compare_chunk_info` (used by `run_deterministic_check`) to take
  the full `GraphCase` so its failure string now includes the seed, the
  encoded case spec, and the reproduce command — matching what the acyclic
  failure markdown already provided.
- Update README repo layout, the reproduce command snippet, and switch the
  test-run examples to `--workspace`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

The fuzz workflow used to run only `acyclic_output_fuzz` via a per-crate
manifest path, so `deterministic_output_fuzz` would never run in CI. Switch
the cargo invocation to `--workspace`, rename the workflow / job / log /
artifact / issue title to drop the "acyclic" qualifier, and rename the
workflow file to match.

Also give `compare_chunk_info` (used by `run_deterministic_check`) the same
`## Failed Seed` ... `### Generate Fixture` markdown envelope the acyclic
failure formatter already produces, so the existing tracking-issue regex
(`## Failed Seed[\s\S]*?### Generate Fixture …`) captures determinism
failures unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>