luci-app-ssr-plus: add optional Xray inbound TCP congestion control

[vsmirn0v]

Problem

The existing custom_tcpcongestion selector only configures outbound sockets. The client-facing Xray transparent listener therefore continues to use the kernel default. On an affected routed/WireGuard path, outbound BBR alone left client downloads around 6 Mbit/s; configuring inbound BBR improved a real transparent-client download to about 57 Mbit/s. This is an environment-specific result, not a claim that BBR is always faster.

Change

• Add a per-node Inbound TCP congestion control selector for Xray's transparent and SOCKS TCP listeners, separate from the existing outbound option.
• Keep System default as the default; do not hard-code BBR or change sysctls.
• Offer only algorithms currently reported by the kernel. If a saved algorithm becomes unavailable, warn and keep the system default rather than fail the listener.
• Hide the option when Xray is unavailable or Mihomo is selected.
• Preserve transparent followRedirect settings and leave UDP-only listeners unchanged.
• Add translations, regression tests and a small Lua 5.1 CI workflow.

Validation

• Generator regressions: absent/empty selection, BBR/CUBIC/Reno, TCP versus UDP, transparent versus SOCKS-only/no-listener configurations, unavailable algorithms, missing procfs, and independent outbound selection.
• Execute the actual CBI declaration to check its UCI key, defaults, choices, visibility and node dependency.
• Tests pass on desktop Lua 5.5 and OpenWrt Lua 5.1.
• Fixtures using actual OpenWrt LuCI modules pass xray run -test with Xray 26.7.11 for both default and BBR configurations, without reading production UCI or starting listeners.
• The underlying inbound-BBR change was verified previously on real transparent client sockets with ss -tin; the new UI is covered by CBI tests, not a browser rendering test.

The new Lua 5.1 CI workflow passed for this PR's head commit.

The README includes manual Save & Apply / generated-config / live-socket acceptance steps. This PR is independent of the UDP/443 policy proposal in #2051.