fix(lab-runner): one currency verdict for materialization staleness, and two fail-open bugs it exposed (#10552)

* feat(lab-contract): one currency verdict for materialization staleness

Adds crates/contracts/homeboy-lab-contract/src/materialization_currency.rs:
Currency (Current/Stale/Unknown), CurrencyEvidence (which evidence a
verdict rests on and whether it can prove byte equality), and comparators
that fail closed to Unknown when either side of a comparison is missing.

Adopts it at the two decision sites where the fail-open was real:

- rig_materialization::with_lab_package_freshness reported `verified` for
  a source whose current revision could not be read at all, because the
  install-time revision was copied into the current-revision slot and then
  compared with itself. homeboy_rig::package_evidence_from_metadata already
  reported `unknown` for the same input. The two now agree.
- runtime_overlay_freshness reported `Fresh` whenever a git probe failed,
  because every probe failure collapsed into commits_behind: None and fell
  through the `> 0` test onto the fresh branch. New UnknownProbeFailed
  status; stale stays false so nothing newly warns or hard-fails.

Refs #10309

* fix(lab-contract): pass digests as str slices in compare_identities

* style: cargo fmt --all

---------

Co-authored-by: chubes-bot <266378653+homeboy-ci[bot]@users.noreply.github.com>

Author: chubes4

crates/contracts/homeboy-lab-contract/src/lib.rs

@@ -9,6 +9,7 @@
 pub mod agent_task_config;
 pub mod agent_task_outcome;
 pub mod env_materialization_plan;
+pub mod materialization_currency;
 pub mod notification_payload;
 pub mod notification_route;
 pub mod path_materialization;