store: replace undersized header.head instead of zeroing it

[rearden-grok]

Summary

H2: open-grow of an undersized single-gen header.head used to zero_range the live OA file. A crash between zero and reinsert left an empty table that the next open treated as valid (silent index wipe).

Open now reads occupied slots, unlinks the OA file, and creates a new one at the create target (.mlt kept). Crash after unlink is a missing header.head (reindex), not a punched live file.

Test plan

• cargo test -p rbitcoin-store rewrite_to_slots_replaces_file_keeps_keys
• cargo test -p rbitcoin-store header_head_open_grows_undersized_single_gen
• cargo test -p rbitcoin-store header_head_rolls
• Required Actions: fmt, deny, clippy, ast-grep, test, windows, macos, multinode, coverage

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