devex: minor deferred polish (doctests, --format json, binding diagnostics export, ci-timings)

[P4suta]

Small DevEx items deferred during the diagnostics / inner-loop work, collected so they aren't lost:

• Doctests no_run → real (done in test(doc): make the front-door + pandoc examples executed doctests #96) — convert no_run doc examples to executed doctests where feasible (currently several are compile-only).
• --format json for render / kinds — the aozora check subcommand has the {human,json,short} format flag; render and kinds do not yet expose a machine format.
• Binding diagnostics_human exports — expose the human/graphical diagnostic rendering through the language bindings (py / wasm / ffi), not just the CLI.
• ci-timings xtask extension — surface per-gate timings for the parallel pre-push pipeline.

Source: DevEx program state (branch feat/devex-inner-loop). Split into individual issues if any grows.

[P4suta]

Status sync after the inner-loop merge (feat/devex-inner-loop → main as 13e56db):

• ✅ Doctests — done in test(doc): make the front-door + pandoc examples executed doctests #96 (front-door + pandoc examples are now executed doctests).
• ⬜ --format json for render/kinds — still open; only check/doc have DiagFormat today.
• ⬜ Binding diagnostics_human exports — still open (no py/wasm/ffi export yet).
• ⬜ ci-timings xtask — still open.

The 3 remaining items are now unblocked (their infra is on main); leaving this open to track them.