refactor(ecstore): extract read IO primitives to set_disk::core::io_primitives (backlog#820) (#4285)

* refactor(ecstore): extract read IO primitives to set_disk::core::io_primitives (backlog#820)

P5 step 1 of the SetDisks God-Object split (tracking backlog#815, issue
backlog#820). Relocate the module-level low-level read/erasure primitives out of
the 5,898-line set_disk/read.rs into a new core/io_primitives.rs module home:

- Metadata-fanout quorum machinery (MetadataQuorumAccumulator,
  MetadataFanoutDiagnostics/Observation, MetadataEarlyStopDecision,
  MetadataCacheLookup).
- Bitrot reader scheduling/creation (BitrotReaderSetup and the
  create_bitrot_readers_* / schedule / fill helpers).
- Shard-cost classification, read-repair heal dedup, and codec-streaming
  reader helpers.

Pure move + visibility adjustment, zero logic change. The moved block is
byte-identical to the pre-move read.rs source modulo the module header swap
(use super::* -> use super::super::*) and item visibility widening
(module-private / pub(super) -> pub(in crate::set_disk) so read.rs still
reaches them). read.rs's impl SetDisks body and imports are byte-identical to
before; mod.rs only gains 'mod core;' and repoints two
GetCodecStreamingReaderBuildOutcome references to the new path.

Verification:
- cargo check / clippy -D warnings -p rustfs-ecstore --all-targets: clean
- cargo test -p rustfs-ecstore --lib: 1840 passed, 0 failed
- normalized + token-stream diff of moved block vs original: identical modulo
  visibility tokens and rustfmt signature re-wrapping

* fix(arch): allow io_primitives as READ_REPAIR_HEAL_CACHE owner (backlog#820)

The READ_REPAIR_HEAL_CACHE owner-path guard in
check_architecture_migration_rules.sh pinned the cache to
set_disk/read.rs. P5 step 1 relocated the cache (and its accessor
helpers) to set_disk/core/io_primitives.rs, so allow that path too.
Guard intent is unchanged: the cache stays behind the ECStore set-disk
read-owner helpers.

---------

Co-authored-by: houseme <housemecn@gmail.com>

scripts/check_architecture_migration_rules.sh

@@ -3869,7 +3869,7 @@ fi
   rg -n --with-filename '\bREAD_REPAIR_HEAL_CACHE\b' \
     crates rustfs fuzz \
     --glob '*.rs' |
-    rg -v '^crates/ecstore/src/set_disk/read\.rs:' || true
+    rg -v '^crates/ecstore/src/set_disk/(read|core/io_primitives)\.rs:' || true
 ) >"$ECSTORE_READ_REPAIR_CACHE_BYPASS_HITS_FILE"
 
 if [[ -s "$ECSTORE_READ_REPAIR_CACHE_BYPASS_HITS_FILE" ]]; then