docs: state host kernel requirements in install.md

[markovejnovic]

Part of #90. Adds a requirement row and a Host Kernel section naming CVE-2026-99999, CVE-2026-53359 and CVE-2026-46316, with where to check for the fix. No provisioning step changes, so setup.sh and provision-kvm-host.sh are untouched.

Written by Claude Code.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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[github-actions]

Test Results

  3 files   -   1  200 suites   - 27   3m 18s ⏱️ -5s
484 tests  - 191  484 ✅  - 190    0 💤 ±0  0 ❌  - 1 
960 runs   - 191  484 ✅  - 190  476 💤 ±0  0 ❌  - 1 

Results for commit e08897c. ± Comparison against base commit 3ac24e1.

This pull request removes 191 tests.

hyper-suidhelper::config_network ‑ absent_network_table_means_disabled
hyper-suidhelper::config_network ‑ partial_network_table_without_uplink_disables_not_bricks
hyper-suidhelper::config_network ‑ present_table_with_only_uplink_defaults_clone_pool
hyper-suidhelper::config_seam ‑ override_path_is_consulted_through_the_real_lexical_gate
hyper-suidhelper::config_uid_gid_range ‑ absent_range_yields_default
hyper-suidhelper::config_uid_gid_range ‑ min_exceeds_max_always_rejected
hyper-suidhelper::config_uid_gid_range ‑ valid_range_accepted
hyper-suidhelper::config_uid_gid_range ‑ valid_range_round_trips_via_toml
hyper-suidhelper::config_uid_gid_range ‑ zero_min_always_rejected
hyper-suidhelper::e2e_argv ‑ blockdev_getsz_argv_and_parse_as_root
…