feat(ENG-306): Add Seat data structure, sector memory model, and wide immediate support

[alnoki]

Changes

1. Define Seat, Order, and StackNode structs with
   fields, and add NodeTag discriminant enum and Sector
   type (max-sized byte buffer) in the interface crate
2. Add wide!() constant kind to the macro system for
   64-bit lddw immediates (ConstantKind::Wide, parser,
   expander with _WD suffix, backed by existing
   Constant::Wide in the build crate)
3. Rename market_data_header to market_header and
   market_data_bytes to market_sectors_start across
   interface, assembly, .tex specs, and tests
4. Add IB_MARKET_SECTORS_START_PTR_WD wide immediate
   using MM_INPUT_START for absolute SBPF pointer
   initialization of MarketHeader.next
5. Add sectors, seats, and orders docs pages with ASCII
   layout diagrams, <Include> regions, and sidebar
   entries
6. Add input buffer subsection to markets docs page
   covering the dynamic offset paradigm
7. Update docs-review skill to verify sector layout
   diagrams against struct definitions and enforce
   reference link placement

[linear]

ENG-306