ttl: implement output blob persistence and output_blob_ref population

[kdy1]

Summary

Build stores metadata but leaves output_blob_ref empty and does not persist task outputs in blob storage.

Evidence

Contract Evidence:

• docs/project-ttl.md:141
• docs/project-ttl.md:152
• cmds/ttlc/internal/compiler/service.go:213

Current Gap

Build stores metadata but leaves output_blob_ref empty and does not persist task outputs in blob storage.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Persist task output payloads to configured blob storage.
• Populate and round-trip output_blob_ref in cache records.
• Validate corruption handling for blob decode/read failures.
• Ensure blob writes/reads follow workspace security constraints.

Test Scenarios

• Successful build persists blob and sets non-empty output_blob_ref.
• Cache hit retrieves output via blob reference.
• Corrupted blob entry triggers safe failure and recompute path.

Out of Scope

• Distributed blob stores.
• Content-addressable deduplication optimizations.