F4: Surface RX seq_num + TSF low on <devourer-stream>

[josephnef]

Summary

Both fields are already on the RX descriptor: seq_num is parsed at FrameParser.cpp:98, tsfl was one commented-out line at line 129. The FEC layer (#86 / #87) and any latency-measurement consumer want both visible; this PR surfaces what the chip already gives us.

Changes

• src/FrameParser.h — add uint32_t tsfl to rx_pkt_attrib alongside the existing seq_num.
• src/FrameParser.cpp — uncomment the TSFL parser and drop the bogus (byte) cast (the macro reads all 32 bits of pdesc+20 as a u32, not a byte — verified against rtl8812a_recv.h):

  - /* pattrib.tsfl=(byte)GET_RX_STATUS_DESC_TSFL_8812(pdesc); */
  + pattrib.tsfl = GET_RX_STATUS_DESC_TSFL_8812(pdesc);

• demo/main.cpp — extend the <devourer-stream> printf with seq=%u tsfl=%u. Optional fields; PR Phy-level soft metrics on stream lines + BER-vs-SNR analyser #84's regex pattern in stream_rx.py / tun_p2p.py / corruption_analysis.py already tolerates them via the same pass-through approach used for rssi/evm/snr.

What this enables (out of scope for this PR — just data surfacing)

• FEC RX side can dedup by chip-side seq before feeding the codec, so air-level retransmissions stop double-counting at the codec.
• One-way latency measurement by diffing TSF against the host clock at TX time — a building block for the F5 TX-RPT goodput numbers and any adaptive --fec-overhead loop.

Test plan

• cmake --build build -j clean
• <devourer-stream> lines on master now carry seq + tsfl fields; existing Python consumers tolerate the additions via their existing regex pass-through (no Python-side change required).
• Reviewer to run an existing tun_p2p bench and confirm the new fields appear without disturbing throughput / loss numbers.

Second in the five-feature C++ series. Followed by:

• F3 — selectable stream-carrier rate/BW (uses F1's HT-MCS unlock + this PR's seq/tsfl plumbing for dup detection)
• F5 — C2H TX-RPT parser + REG_FIFOPAGE_INFO queue-depth poll
• F2 — BB-dbgport per-subcarrier IQ spike (research)

Predecessor: F1 (#88).

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