test(#496): Add FPGA stub backend validating vtable pluggability

[justinjoy]

Summary

• Add test-only naive row-store FPGA stub backend to validate wl_compute_backend_t vtable is truly pluggable
• Implement all 7 vtable slots with wl_fpga_session_t embedding wl_session_t base as first field
• Row-store evaluator handles all 9 universal operators (0-8), skips columnar-specific ops (9-11)
• Fixed-point iteration for recursive strata with delta tracking
• 11 test cases covering lifecycle, queries (passthrough, join, filter, TC), delta callbacks, and backend-specific op handling
• Interface gap findings documented in fpga_backend.c header comment

Key validation results

• Session embedding contract (C11 struct casting): works cleanly
• Intern sharing via plan->intern: works across backends
• Delta naming convention ($d$): backend-agnostic
• Plan-gen coupling: exec_plan_gen.c unconditionally emits K_FUSION/LFTJ/EXCHANGE (documented as finding)
• wl_easy.h hardcodes columnar backend (documented as finding)

Files (all test-only, no changes to wirelog/)

• tests/fpga_backend.h — header
• tests/fpga_backend.c — implementation (~1130 LOC)
• tests/test_fpga_backend.c — 11 tests
• tests/meson.build — test registration

Closes #496

Test plan

• 135/135 tests pass (134 existing + 1 new FPGA test)
• FPGA lifecycle: create/destroy/insert/remove(-1)/snapshot-empty
• Simple passthrough, join, filter queries produce correct results
• Transitive closure reaches fixed point via semi-naive iteration
• Delta callback fires with diff=+1 for new tuples
• Backend-specific ops (K_FUSION/LFTJ/EXCHANGE) skipped without crash
• Zero modifications to wirelog/ source files