ci: let the coverage gate see every backend, and document the one it caught

[godofecht]

Found by opening #555, and then immediately proved by merging it.

The gap

check_doc_coverage.py discovered backends by matching *_generator.py, *_codegen.py and *_compiler.py. The BPF target is bpf_target.py, so a whole new compilation target would have merged without the coverage gate asking for a documentation page.

Added _target and _backend to the suffix list, with a test enumerating all five conventions against a temporary tree.

The gate then caught it for real

When #555 merged and I rebased this branch onto main, the checker did exactly what it was built to do:

backend       9/10   covered   exempt   0   problems   0   uncovered   1

1 problem(s):
  backend:bpf_target has no documented home

So this PR also carries docs/language/bpf.md, written from the implementation rather than from the commit messages:

• the CLI, every flag
• the fixed ABI: bpfel triple, 64-bit pointers, and the verifier's 512-byte stack limit
• the section and license metadata the kernel reads, with the actual IR it produces
• the ten symbols the target refuses, because a BPF program runs with no userspace runtime (malloc, free, _Znwm, __cxa_throw, …)
• the entry-point naming rule and why it exists: the target lowers with --llvm so the definition carries the plain Flow name, while flow_export_<name> is a C-backend alias. Searching only for the alias is what made every build on that branch fail for eight runs.
• the toolchain caveat, the same as wasm32: Apple's clang has no BPF backend

Filed under Targets & interop in the sidebar, next to WebAssembly.

Validation

• check_doc_coverage.py: 10/10 backends, every feature covered
• wiki_nav.py consistent, check_doc_links.py clean including fragments
• check_doc_examples.py --check-ledger clean
• Unit suite 1510 passed, 2 skipped
• ruff check src clean