feat(server): parse CSI wire v2 and v3 headers

[clonea1]

The frame parser accepts one magic (0xC5110001) and a fixed 20-byte header, so
a frame carrying transmitter or transmission identity is rejected outright.
That makes it impossible for a node to tell the server anything about WHICH
link a measurement belongs to, or WHICH transmission produced it.

Adds named constants for the three wire versions and dispatches on the magic:

v1  0xC5110001  20-byte header, unchanged
v2  0xC5110008  + transmitter MAC (addr2) at bytes 20..25
v3  0xC511000A  + 802.11 rx_seq (u16 LE) at bytes 26..27

Esp32Frame gains source_mac and rx_seq, both Option, both None for a v1 frame.
A mixed fleet therefore keeps working: older firmware parses exactly as before
and newer firmware simply carries more.

This is the receiver half of the wire change and should land BEFORE any
firmware starts emitting v2/v3, so there is never a moment where a node sends
something the server rejects.

It is also a prerequisite for two follow-on changes that cannot work without
per-frame identity: per-link CSI state keyed on (receiver, transmitter), and
cross-node pairing keyed on (transmitter, rx_seq).

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Rebased onto current main before opening: staged before today's seven merges, so it needed replaying to avoid reading as a revert of them. Clean rebase, no files deleted.

[clonea1]

Mapped this cohort against ADR-345 in #1956. This PR is the root of it —
it is the wire change that lets a measurement say which link it belongs to,
which is ADR-345's whole premise, and ADR-345 measured that on node 2 the AP
was only 34% of frames while the other 66% were peer frames the sink could
not attribute.

One commit, server-side only, no dependencies. Its parser commit is carried
by #1828, #1833 and #1834 because a fork PR cannot be based on another fork
branch; merging this one unblocks all three, and each then drops the
duplicate by patch-id on rebase with nothing to resolve.

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