feat: add delta support (proto + compute_delta/apply_delta_bytes) for CountMin, CountSketch, and HLL

[zzylol]

Summary

Extends the delta-encoding support previously added for DDSketch to three more sketch families, so the runtime can emit and apply sparse deltas for them. Each family gets a *Delta proto message, regenerated prost bindings, and compute_delta(snapshot, threshold) + apply_delta_bytes(&[u8]) on its portable wrapper.

These deltas carry only sketch-internal state (cells / register updates) -- they never had DataPoint-level metric scalars, so there is nothing to drop.

Per family

• CountMinSketch -- additive cell deltas. A cell is carried when its absolute count change meets the threshold. The wire form is the packed cell_rows/cell_cols/d_counts arrays (signed sint64; CMS counters only grow but the signed form covers weighted/decay variants) plus full per-row l1/l2 norm deltas.
• CountSketch -- signed cell deltas in the same packed-array form, plus per-row l2 norm deltas. Heavy-hitter candidate keys (hh_keys) are forwarded when an upstream tracker provides them; the minimal wrapper here leaves them empty.
• HyperLogLog -- lossless register max-updates: a register's new value when it increased, applied on the receiver via register[i] = max(register[i], value). No register update is ever dropped (there is no threshold to apply).

Byte parity

The emitted delta bytes are byte-identical to the Go reference implementation's delta output for identical input (cross-language byte parity). Each *Delta proto matches the Go reference's field numbers and wire encoding exactly, and each family has a golden test asserting the produced bytes match a fixture captured from the Go reference implementation.

Tests

For each family:

• a round-trip test: build a sketch, compute_delta against a snapshot, apply_delta_bytes onto the snapshot, and assert it reconstructs the sketch (including a delta-against-empty case that reconstructs the full window state);
• a byte-parity assertion against a Go-produced golden.

cargo build and cargo test are green (422 lib tests passing).