iter-126: canonical network JSON shape for navigate --with-network and network

[ractive]

Summary

• Fixes the network JSON shape flip. navigate --with-network (and the standalone network command) returned an inconsistent JSON shape: a {entries, …} object on busy pages but a bare array on quiet ones. .results.network.entries / .results.network.total_requests threw cannot index array half the time, and the documented summary fields were unreachable via --jq (which forces detail mode).
• One canonical object on every path. New shared pub(crate) build_canonical_network + merge_summary_fields builders in network.rs. apply_network_controls now returns a single object — {entries, shown, total, truncated, total_requests, total_transfer_bytes, by_cause_type, slowest, timeout_reached, …} — on busy/quiet, detail/summary, --all/default. Summary counts always reflect the full capture, never the truncated/field-projected view.
• Standalone network detail carries summary fields. The detail-mode envelope now merges the summary fields alongside results, so --jq users are no longer cut off from total_requests/total_transfer_bytes/slowest.
• Docs. args.rs help text documents the canonical shape with a bare-array backward-compat note (both the network output line and the navigate --with-network long_about section).

Test plan

• cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace -q — all clean (718 + 279 + … passing, 0 failures).
• cargo run -p xtask -- check-iteration-ready --plan kb/iterations/iteration-126-network-json-shape-consistency.md --base origin/main — 10/10 PASS (with FF_RDP_LIVE_TESTS=1).
• 6 new unit tests incl. a navigate/network parity assertion (network_and_navigate_summary_fields_agree_field_for_field), truncation-math, and empty-page key-presence.
• 3 new mock e2e tests (navigate_with_network_detail_mode_is_object_not_array, ..._all_keeps_object_shape, summary fields in network_detail_shows_requests).
• 3 new live tests in live_126_network_shape.rs covering the quiet/busy shape parity, --all, and standalone network --jq summary carry.
• Dogfooded against live Firefox: example.com (quiet) → {"t":"array","n":1,"truncated":false}; wikipedia (busy, 118 req) → {"t":"array","n":118,"shown":20,"truncated":true} — identical key sets, no shape flip; standalone network --jq returns total_requests: 118, total_transfer_bytes: 2025146.0.

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