feat: support container-confinement service keys (read_only, init, privileged, cap_add, cap_drop, security_opt)

[lesnik512]

Second Bucket A bundle. Adds six per-container container-confinement keys, each an invariant-preserving podman run flag with no change to the single-pod model.

• read_only / init / privileged — booleans, emitted as the bare --read-only / --init / --privileged flag only when true (new _BOOL_FLAGS table).
• cap_add / cap_drop / security_opt — lists, emitted as repeated --cap-add / --cap-drop / --security-opt (reuse the existing _LIST_FLAGS table; item contents pass through verbatim, ${VAR}-interpolated like group_add).

Also in this branch:

• stop_signal / stop_grace_period reclassified out of Bucket A and wired into IGNORED_SERVICE_KEYS (accepted with a warning, not rejected) — they are inert because the generated script force-removes the pod (podman pod rm -f) and never gracefully stops a container.
• test_unsupported_service_key_raises (and a sibling CLI test) retargeted from privileged (now supported) to network_mode (a permanent-reject key).

Design and rationale (single home, not restated here):

• Change/design: planning/changes/2026-07-09.04-container-confinement-service-keys.md
• Stop-key change: planning/changes/2026-07-09.05-ignore-stop-lifecycle-keys.md
• Reclassification decision: planning/decisions/2026-07-09-stop-lifecycle-keys-inert.md

Behavior promoted into architecture/supported-subset.md in the same PR.

Tests: 163 passing at 100% line coverage; just lint-ci clean.

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