[REF] core: replace streaming pipeline with staged batch pipeline

The deduplication core was a three-stage streaming producer/consumer
(scan -> group-by-size -> group-by-hash) orchestrated by a Server struct
that ran the stages concurrently on a threadpool. Coordination relied on
AtomicBool flags polled in busy-wait loops, an Arc<Mutex<Vec>> hand-off
queue, DashMap stores, and a per-file Arc<Mutex<FileState>>. Every stage
had to receive its own hand-cloned Arcs with ad-hoc names, and the
busy-wait branches burned a core spinning while waiting for the producer.

Replace it with a staged batch pipeline over owned collections:
pipeline::run(&Params) drives scan -> group_by_size -> group_by_hash in
sequence, with rayon supplying parallelism. With no shared mutable state
between stages, all the Arc plumbing, the AtomicBool coordination, the
Mutex queue, the per-file lock, server.rs, and the threadpool/dashmap
dependencies are gone. FileInfo becomes plain data and each stage is a
pure, independently testable function.

Behavior is preserved except for three authorized deviations: progress
spinners render sequentially rather than concurrently under -p; the
interactive empty-result message prints once instead of twice; and the
interactive "Duplicate Set X of Y" total now counts the confirmed
duplicate groups shown instead of the internal candidate-hash store size.

This lands as the foundation for the cache, CLI, and TUI work that follows.

Cargo.lock

@@ -256,21 +256,6 @@ dependencies = [
  "memchr",
 ]
 
-[[package]]
-name = "dashmap"
-version = "6.1.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5041cc499144891f3790297212f32a74fb938e5136a14943f338ef9e0ae276cf"
-dependencies = [
- "cfg-if",
- "crossbeam-utils",
- "hashbrown",
- "lock_api",
- "once_cell",
- "parking_lot_core",
- "rayon",
-]
-
 [[package]]
 name = "deduplicator"
 version = "0.3.2"
@@ -279,7 +264,6 @@ dependencies = [
  "bytesize",
  "chrono",
  "clap",
- "dashmap",
  "globwalk",
  "gxhash",
  "indicatif",
@@ -289,7 +273,6 @@ dependencies = [
  "rand",
  "rayon",
  "tempfile",
- "threadpool",
  "unicode-segmentation",
 ]
 
@@ -398,12 +381,6 @@ dependencies = [
  "rustversion",
 ]
 
-[[package]]
-name = "hashbrown"
-version = "0.14.5"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
-
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -531,16 +508,6 @@ version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cd945864f07fe9f5371a27ad7b52a172b4b499999f1d97574c9fa68373937e12"
 
-[[package]]
-name = "lock_api"
-version = "0.4.13"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "96936507f153605bddfcda068dd804796c84324ed2510809e5b2a624c81da765"
-dependencies = [
- "autocfg",
- "scopeguard",
-]
-
 [[package]]
 name = "log"
 version = "0.4.27"
@@ -571,16 +538,6 @@ dependencies = [
  "autocfg",
 ]
 
-[[package]]
-name = "num_cpus"
-version = "1.17.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "91df4bbde75afed763b708b7eee1e8e7651e02d97f6d5dd763e89367e957b23b"
-dependencies = [
- "hermit-abi",
- "libc",
-]
-
 [[package]]
 name = "once_cell"
 version = "1.21.3"
@@ -593,19 +550,6 @@ version = "1.70.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a4895175b425cb1f87721b59f0f286c2092bd4af812243672510e1ac53e2e0ad"
 
-[[package]]
-name = "parking_lot_core"
-version = "0.9.11"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bc838d2a56b5b1a6c25f55575dfc605fabb63bb2365f6c2353ef9159aa69e4a5"
-dependencies = [
- "cfg-if",
- "libc",
- "redox_syscall",
- "smallvec",
- "windows-targets 0.52.6",
-]
-
 [[package]]
 name = "pathdiff"
 version = "0.2.3"
@@ -714,15 +658,6 @@ dependencies = [
  "crossbeam-utils",
 ]
 
-[[package]]
-name = "redox_syscall"
-version = "0.5.13"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "0d04b7d0ee6b4a0207a0a7adb104d23ecb0b47d6beae7152d0fa34b692b29fd6"
-dependencies = [
- "bitflags",
-]
-
 [[package]]
 name = "redox_users"
 version = "0.4.6"
@@ -785,12 +720,6 @@ dependencies = [
  "winapi-util",
 ]
 
-[[package]]
-name = "scopeguard"
-version = "1.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
-
 [[package]]
 name = "serde"
 version = "1.0.219"
@@ -817,12 +746,6 @@ version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0fda2ff0d084019ba4d7c6f371c95d8fd75ce3524c3cb8fb653a3023f6323e64"
 
-[[package]]
-name = "smallvec"
-version = "1.15.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "67b1b7a3b5fe4f1376887184045fcf45c69e92af734b7aaddc05fb777b6fbd03"
-
 [[package]]
 name = "strsim"
 version = "0.11.1"
@@ -884,15 +807,6 @@ dependencies = [
  "syn",
 ]
 
-[[package]]
-name = "threadpool"
-version = "1.8.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d050e60b33d41c19108b32cea32164033a9013fe3b46cbd4457559bfbf77afaa"
-dependencies = [
- "num_cpus",
-]
-
 [[package]]
 name = "unicode-ident"
 version = "1.0.18"