Remediate all lint findings under the canonical golangci-lint config

Fix every finding surfaced by the canonical .golangci.yml with
golangci-lint v2.12.2 (refs #61), behavior-preserving throughout:

- err113: dynamic errors replaced with package-level sentinels and %w
  wrapping; direct comparisons converted to errors.Is
- goprintffuncname: printf-style helpers renamed with an f suffix
  (ui.Writer message methods, cli.ReportErrorf, database.Fatalf,
  vaultik stdoutf) and all call sites updated
- revive: stuttering type names renamed (blob.Handler, blob.WithReader,
  blob.ChunkPosition, storage.URL, storage.Info), doc comments added,
  unused parameters blanked, package comments added
- contextcheck/noctx: ctx threaded through blob.Packer
  (AddChunk/Flush/FinalizeBlob/PackChunks) and scanner call sites;
  context-aware exec and sql variants used
- funlen/cyclop/gocognit/nestif/dupl: oversized or duplicated
  functions split into focused helpers across production and test code
- paralleltest/tparallel/thelper/usetesting/testpackage: tests
  parallelized where safe (global log.Initialize kept in the serial
  phase), helpers marked, t.TempDir adopted, external test packages
  where only exported API is used
- gosec: integer conversions clamped or justified, header timeouts
  added, remaining findings suppressed with per-site justifications
- mnd/goconst/lll/wsl_v5/nlreturn/noinlineerr/errcheck and other
  mechanical findings fixed directly

Remove the deprecated log.LogOptions alias (callers migrated to
log.Options). make check is green.

internal/chunker/fastcdc.go

@@ -1,6 +1,7 @@
 package chunker
 
 import (
+	"errors"
 	"io"
 	"math"
 	"sync"
@@ -27,6 +28,8 @@ type ReusableChunker struct {
 }
 
 // reusableChunkerPool pools ReusableChunker instances to avoid allocations.
+//
+//nolint:gochecknoglobals // process-wide object pool by design
 var reusableChunkerPool = sync.Pool{
 	New: func() any {
 		return &ReusableChunker{}
@@ -35,6 +38,8 @@ var reusableChunkerPool = sync.Pool{
 
 // bufferPools contains pools for different buffer sizes.
 // Key is the buffer size.
+//
+//nolint:gochecknoglobals // process-wide buffer pools by design
 var bufferPools = sync.Map{}
 
 func getBuffer(size int) []byte {
@@ -45,9 +50,18 @@ func getBuffer(size int) []byte {
 			return &buf
 		},
 	})
-	pool := poolI.(*sync.Pool)
 
-	return *pool.Get().(*[]byte)
+	pool, ok := poolI.(*sync.Pool)
+	if !ok {
+		panic("bufferPools holds a non-pool value")
+	}
+
+	buf, ok := pool.Get().(*[]byte)
+	if !ok {
+		panic("buffer pool holds a non-buffer value")
+	}
+
+	return *buf
 }
 
 func putBuffer(buf []byte) {
@@ -55,7 +69,11 @@ func putBuffer(buf []byte) {
 
 	poolI, ok := bufferPools.Load(size)
 	if ok {
-		pool := poolI.(*sync.Pool)
+		pool, isPool := poolI.(*sync.Pool)
+		if !isPool {
+			panic("bufferPools holds a non-pool value")
+		}
+
 		b := buf[:size]
 		pool.Put(&b)
 	}
@@ -69,11 +87,21 @@ type FastCDCChunk struct {
 	Fingerprint uint64
 }
 
-// AcquireReusableChunker gets a chunker from the pool and initializes it for the given reader.
-func AcquireReusableChunker(rd io.Reader, minSize, avgSize, maxSize int) *ReusableChunker {
-	c := reusableChunkerPool.Get().(*ReusableChunker)
+// bufSizeFactor sizes the internal read buffer relative to the maximum
+// chunk size so a full chunk plus read-ahead always fits.
+const bufSizeFactor = 2
 
-	bufSize := maxSize * 2
+// AcquireReusableChunker gets a chunker from the pool and initializes it
+// for the given reader.
+func AcquireReusableChunker(
+	rd io.Reader, minSize, avgSize, maxSize int,
+) *ReusableChunker {
+	c, ok := reusableChunkerPool.Get().(*ReusableChunker)
+	if !ok {
+		panic("reusableChunkerPool holds a non-chunker value")
+	}
+
+	bufSize := maxSize * bufSizeFactor
 
 	// Reuse buffer if it's the right size, otherwise get a new one
 	if c.buf == nil || cap(c.buf) != bufSize {
@@ -112,37 +140,6 @@ func (c *ReusableChunker) Release() {
 	reusableChunkerPool.Put(c)
 }
 
-func (c *ReusableChunker) fillBuffer() error {
-	n := len(c.buf) - c.cursor
-	if n >= c.maxSize {
-		return nil
-	}
-
-	// Move all data after the cursor to the start of the buffer
-	copy(c.buf[:n], c.buf[c.cursor:])
-	c.cursor = 0
-
-	if c.eof {
-		c.buf = c.buf[:n]
-
-		return nil
-	}
-
-	// Restore buffer to full capacity for reading
-	c.buf = c.buf[:c.bufSize]
-
-	// Fill the rest of the buffer
-	m, err := io.ReadFull(c.rd, c.buf[n:])
-	if err == io.EOF || err == io.ErrUnexpectedEOF {
-		c.buf = c.buf[:n+m]
-		c.eof = true
-	} else if err != nil {
-		return err
-	}
-
-	return nil
-}
-
 // Next returns the next chunk or io.EOF when done.
 // The returned Data slice is only valid until the next call to Next.
 func (c *ReusableChunker) Next() (FastCDCChunk, error) {
@@ -170,6 +167,37 @@ func (c *ReusableChunker) Next() (FastCDCChunk, error) {
 	return chunk, nil
 }
 
+func (c *ReusableChunker) fillBuffer() error {
+	n := len(c.buf) - c.cursor
+	if n >= c.maxSize {
+		return nil
+	}
+
+	// Move all data after the cursor to the start of the buffer
+	copy(c.buf[:n], c.buf[c.cursor:])
+	c.cursor = 0
+
+	if c.eof {
+		c.buf = c.buf[:n]
+
+		return nil
+	}
+
+	// Restore buffer to full capacity for reading
+	c.buf = c.buf[:c.bufSize]
+
+	// Fill the rest of the buffer
+	m, err := io.ReadFull(c.rd, c.buf[n:])
+	if errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF) {
+		c.buf = c.buf[:n+m]
+		c.eof = true
+	} else if err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func (c *ReusableChunker) nextChunk(data []byte) (int, uint64) {
 	fp := uint64(0)
 	i := c.minSize
@@ -198,6 +226,8 @@ func (c *ReusableChunker) nextChunk(data []byte) (int, uint64) {
 }
 
 // 256 random uint64s for the rolling hash function (from FastCDC paper)
+//
+//nolint:gochecknoglobals // immutable FastCDC gear lookup table
 var table = [256]uint64{
 	0xe80e8d55032474b3, 0x11b25b61f5924e15, 0x03aa5bd82a9eb669, 0xc45a153ef107a38c,
 	0xeac874b86f0f57b9, 0xa5ccedec95ec79c7, 0xe15a3320ad42ac0a, 0x5ed3583fa63cec15,