chore: unpin soldr in .clud/settings.json so clud stops downgrading the global soldr

[zackees]

Context

.clud/settings.json on main pins soldr:

"optimize": {"rust": {"install_soldr": true, "soldr_version": "0.7.11", "use_soldr_shims": true}}

• The pin was added on 2026-06-16 in 31e28e2 ("optimize"). clud optimize wrote it at a time when its --soldr-version flag had a hardcoded default of 0.7.11 (zackees/clud PR fix(zccache): bound ensure_running with a 30s timeout (#346) #349, commit 83ea446). Nobody chose that version on purpose. soldr 0.7.11 was released on 2026-04-23, and 0.7.55 was already out when the pin was written.
• The pin was dormant at first because clud only installed soldr when it was missing. That changed in zackees/clud commit 09b4fc0 (2026-08-25, clud issue pybuild: upgrade PyO3 to 0.29 and remove Windows import-library workaround #1025). Since then, clud's soldr activation (crates/clud-bin/src/soldr_activate.rs) checks an explicit pin on every launch. If the installed soldr doesn't match the pin, it runs uv tool install --force soldr==0.7.11.
• As a result, starting clud in this repo downgrades the user's global soldr, and every other project on the machine shares that soldr. On 2026-09-14, a clud launch in fbuild replaced soldr 0.9.15 with 0.7.11.

Proposal

Delete only the soldr_version key from .clud/settings.json. Keep install_soldr: true and use_soldr_shims: true. With no pin, clud floats: it uses whatever soldr is already installed and installs the latest soldr only if none is present.

Add a small guard test so a later clud optimize run can't quietly bring the pin back.

Out of scope: the uv.lock dev-group entry for soldr (currently 0.8.30). It only sets soldr inside the project venv (pyproject.toml requires >=0.8.19) and never changes the global uv tool, so it doesn't cause this downgrade.

Acceptance criteria

• .clud/settings.json has no soldr_version key at any depth, and install_soldr / use_soldr_shims are still true.
• A guard test in ci/ fails while the pin is present (RED) and passes after it is removed (GREEN). The PR includes both outputs.
• CI runs the guard.

Decisions

• Float instead of bumping the pin. Any fixed version goes stale and then downgrades every newer global install. Floating matches clud's default behavior.
• Separate PR. PR fix(esp32): build what the ini pins, and match PlatformIO's ESP32-S3 size (#1432) #1434 (commit ab6569c) makes the same settings change, but it's bundled with unrelated ESP32-S3 work. This standalone fix doesn't have to wait for that PR. The settings hunk is identical in both, so they merge cleanly in either order.
• The guard is a stdlib unittest in ci/, like ci/test_rust_toolchain_pins.py, so it runs with uv run --no-project and no extra dependencies.