error codes: migrate storage call sites to STG-0xx codes #359

Background

Part of #277's rollout, step 3. Migrate storage/file-format call sites to real STG-0xx codes (27 documented codes in docs/ERROR_REFERENCE.md).

Scope

bail!/anyhow! call sites across the storage layer (all already anyhow-based, no blocker like PRS's):

• src/storage/persistent_facts.rs
• src/storage/backend/file.rs
• src/storage/mod.rs
• src/storage/btree_v6.rs / src/storage/btree.rs
• src/storage/packed_pages.rs
• src/storage/cache.rs
• src/storage/backend/memory.rs

Work item

Migrate each site to bail_coded!/err_coded! with its real STG-0xx code; add regression tests asserting the specific code comes back through the public API (e.g. opening a corrupt/truncated .graph file).

Depends on: #357 (merged)
No dependency on the PRS issue — independent module, can run in parallel.
Part of #277