Simplify progress tracking + Unify Task ID tracking + Legacy Interface (#726)

Currently, progress tracking is split between callback-driven and
snapshot-driven paths, making session and task wiring across xet_data,
xet_pkg, hf_xet, and git_xet harder to keep consistent. This PR moves
upload/download progress to a polling snapshot model backed by atomics.
It also switches task identifiers to a UniqueID common with the progress
tracking throughout the session APIs.

This PR also updates the rate estimation to use the lighter weight
exponentially weighted moving averages model, so this can be done at a
low level.

To preserve compatibility for existing callback consumers,
callback-oriented upload/download progress tracking APIs are moved under
xet_pkg::legacy and bridged from polling snapshots via a callback based
updaters. hf_xet and git_xet are updated to use that legacy bridge
layer, so current integrations keep working until everything is fully
switched over to the XetSession method.

wasm/hf_xet_wasm/Cargo.lock

@@ -3219,16 +3219,6 @@ version = "1.14.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f8c1ae7cc0fdb8b842d65d127cb981574b0d2b249b74d1c7a2986863dc134f71"
 
-[[package]]
-name = "ulid"
-version = "1.2.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "470dbf6591da1b39d43c14523b2b469c86879a53e8b758c8e090a470fe7b1fbe"
-dependencies = [
- "rand 0.9.2",
- "web-time",
-]
-
 [[package]]
 name = "unicase"
 version = "2.9.0"
@@ -4201,7 +4191,6 @@ dependencies = [
  "tokio",
  "tokio-util",
  "tracing",
- "ulid",
  "url",
  "walkdir",
  "xet-client",