* perf(ecstore): seek encrypted multipart Range GETs to the covering part boundary on the Legacy rio v1 backend
Phase A of https://github.com/rustfs/backlog/issues/1316. Encrypted Range GETs on the default (non rio-v2) backend previously planned storage_offset=0, storage_length=oi.size and discarded the decrypted prefix, so a small Range on a large SSE object read, erasure-decoded, and decrypted the whole object. Eligible multipart objects now seek to the covering part boundary using only the per-part size/actual_size metadata facts; the multipart decrypt reader already handles streams starting at any part boundary, so rio and the on-disk format are untouched. Single-part objects, compressed payloads, defective parts tables, and zero-length ranges keep the previous full read, and RUSTFS_ENCRYPTED_RANGE_SEEK=false restores it globally. A new histogram rustfs_get_encrypted_range_read_amplification plus a full|part_seek path counter record the physical/plaintext amplification at the ReadPlan decision point.
* fix(ecstore): guard encrypted multipart range seeks
* fix(io-metrics): align encrypted range metric names with the rustfs_io_ prefix
Every other metric in rustfs-io-metrics uses the rustfs_io_ prefix; the
two encrypted-range-seek metrics were the only exception. Rename before
first release so dashboards never see the unprefixed names.
Zhengchao An