Blocks/Digital : porting header files for digital block from 3.x to 4.0

[KrxGu]

Kickoff: GR3 → GR4 Digital

First slice adds LFSR kernels using the processOne() + start()/stop() pattern (no heavy classes, no custom ctors). Design follows KISS / C++23 STL.

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Proposed layout (feedback welcome)

blocks/digital/include/digital/
  core/       # Lfsr, Crc, Scrambler, Constellation (data+strategy), Enums
  mapping/    # chunks→symbols, constellation enc/soft-dec, diff, slicers, map
  timing/     # MM clock, symbol sync, Costas/FLL/PFB
  measure/    # SNR/EVM/probes
  equalizer/  # CMA/LMS/NLMS + LE/DFE
  ofdm/       # CP, serializer, alloc, eq, chan-est, sync
  packet/     # header formats, hdr gen/parse, sink, demux
  misc/       # correlators, burst shaper, protocol/HDLC
  compat/     # 3.x shims + using-aliases

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Rationale

• Single surface: verify kernels by testing processOne() directly.
• Small headers, STL-first: easy to reason about; easy to SIMD/bulk-extend later.
• No inheritance/vtables: lowers per-sample overhead.