spec(policy): l2_generation -- gen/json/ is the backend directory of t27c gen-json #10079

Problem

PR #10064 (issue #10050) adds t27c gen-json and commits its output as gen/json/igla/own_model_board.json. The L2 GENERATION check fails that PR:

rule: gen/c/policy/l2_generation.c at origin/master
HAND-MADE COPY  gen/json/igla/own_model_board.json: a backend directory t27c does not write

The rule (specs/policy/l2_generation.t27) pairs every gen/<backend>/ directory with the t27c subcommand that writes it, and json is not in that table. The plumbing runs the BASE's copy of the rule, so a PR cannot loosen the rule that judges it. That is by design, and it means the table entry has to land on master first.

Proposal

Add one entry to the table: json -> gen-json. Then a gen/json/<stem>.json copy is checked the way every other copy is: t27c gen-json specs/<stem>.t27 must reproduce it byte for byte, and a later edit to the spec that leaves the copy behind fails as a STALE COPY.

Boundary

• specs/policy/l2_generation.t27
• gen/c/policy/l2_generation.c

User Scenarios

• Given this rule on master, when PR feat(t27c): gen-json -- the IGLA board event a spec states, as one JSON object (Closes #10050) #10064 runs the L2 check, then gen/json/igla/own_model_board.json is checked against t27c gen-json specs/igla/own_model_board.t27 instead of being refused as hand-made.
• Given this rule, when a PR adds gen/jsonl/a.json, then it is still refused: unknown backend directory.

Requirements

• FR-001: BACKENDS and SUBCOMMANDS MUST gain the pair json / gen-json at the same index, pinned by the the_backend_table_is_pinned invariant.
• FR-002: gen/c/policy/l2_generation.c MUST be regenerated by t27c gen-c, not edited by hand.

Success criteria

• The spec's tests pass on the lab, with no vacuous passes.
• gen/c/policy/l2_generation.c is byte-identical to t27c gen-c specs/policy/l2_generation.t27.
• After this merges, the L2 check on feat(t27c): gen-json -- the IGLA board event a spec states, as one JSON object (Closes #10050) #10064 passes.