Restore a private marker's direction (and highlight_duration) when a
project is reopened — _create_link_from_topology_data previously dropped
them, silently reverting rx/tx markers to "both".
Reject tx/rx direction on marker definitions with HTTP 409: a definition
auto-selects its capture node per link and direction is relative to that
node, so a fixed tx/rx has no stable project-wide meaning. Per-link
markers still support tx/rx; only the project-wide definition is restricted
to "both" (the default).
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