fix(orchestrator): make treehash-mapping cache write best-effort (audit #8)

[sbalabanov]

Summary

The treehash-mapping Put is a pure cache optimization (maps build description to treehash to skip workspace materialization on future requests), but its failure was incorrectly failing the entire request after a successful graph computation.

• Make treehash-mapping Put best-effort: on error, log a structured warning and continue serving the computed result
• Add inline comment documenting the two-tier cache write policy (identity-bearing writes are load-bearing; pure-optimization writes are best-effort)
• Add table-driven tests pinning both behaviors: treehash Put failure still succeeds, graph blob write failure still fails

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