fix(code-index): never join clone backfill from a freshness read

[ScriptedAlchemy]

Reproduction

On the local current-master daemon, both tracedecay tool status and tracedecay status blocked past 90/30 seconds while clone backfill ran. Attaching gdb during the blocked request gave the exact stack:

std::sys::sync::mutex::futex::Mutex::lock_contended
LatestCodeTextGenerationV1::clone_index_status
dashboard_freshness_read

text_projection_needs_work already states and implements the invariant: a read probe must not queue behind an advance because backfill holds the slot for its whole bounded slice; contention itself means work is in progress. clone_successor_progress, called by clone_index_status, still used blocking lock_slot().

Checked #1577 first: it changes clone codec/decode, not clone_index_status, clone_successor_progress, or this lock.

Fix

clone_successor_progress uses try_lock and returns an explicit Busy read state. clone_index_status maps that to the existing typed status:

Unavailable { reason: "clone-index status is being updated" }

No guessed progress counters and no new wire state. The artifact/census read happens only after the slot snapshot succeeds.

Behavior test

dashboard_freshness_does_not_join_a_clone_backfill_slice drives a mounted registry, obtains ready text owners, holds the backfill slot, then calls the public dashboard freshness surface. It must return the typed busy clone status within 100 ms. The old code deadlocks on the held slot and fails the timeout.

Focused clone-status tests: 2/2. All dashboard-freshness runtime tests: 3/3. rustfmt and IDE lints clean.

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