Align native Xray with xray-core; drop dead knobs; split admin app.js

Fix the two reliability problems in the in-process Xray emulator by matching
XTLS/Xray-core's transport semantics:

- XHTTP upload queue: rewrite as a faithful port of xray-core's uploadQueue
  (bounded channel + sequence reorder heap). Packet-up POSTs are now acked
  immediately on buffering instead of blocking until the tunnel reader consumes
  them. The old block-until-consumed behavior throttled the uplink to the
  reassembly rate and deadlocked against the client's concurrent-POST limit,
  which showed up as "download a burst, stall, repeat" on video/large downloads.
- Mux: dial the backend and pump uplink on a per-session goroutine fed by a
  bounded channel (mirrors xray-core's per-session buffered pipe). Previously the
  dial and backend writes ran inline in the shared read loop, so one slow target
  or backpressured session stalled every other muxed session.
- XHTTP download writer: flush every write (matches httpServerConn.Write) instead
  of batching behind a 2ms/32KB window.
- XHTTP: enforce a single download (stream-down) per session to stop two GETs
  from splitting the decoded stream and corrupting the tunnel.
- Fix a close-of-closed-channel race in the mux session teardown (sync.Once).

Remove the now-inert XHTTP tuning knobs (xhttp_queue_timeout_ms, xhttp_flush_ms,
xhttp_flush_bytes) from the backend struct and the admin panel UI.

Split admin/assets/app.js into ordered classic-script modules under
admin/assets/js/ for maintainability. The concatenation is byte-identical to the
old file and load order is preserved via defer, so behavior is unchanged.

Add regression tests for the mux head-of-line stall and the out-of-order
packet-up burst-stall; add golang.org/x/text to go.mod so tests build.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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