Make the stale-packet gate opt-in per latest-wins topic (it's over-broad)

[rolker]

Make the stale-packet gate opt-in per latest-wins topic

The stale-packet gate added in b7ae191 (import PR #33, deployed on the
2026-06-29 BizzyBoat deployment) ships global default-ON and keyed
per-destination-topic. The field found it over-broad: it is correct only for
single-stream latest-wins topics (heartbeat/video/costmap — which it fixed) but
wrong for:

• every-sample-matters topics (raw sonar sonar_image_*, pointclouds) — each
  ping is unique data, so an out-of-order drop is a gap, not a dedup;
• multiplexed topics like /tf — one topic carries many frames; a late /tf
  for frame A is dropped because a newer /tf for frame B advanced the
  per-topic high-water, dropping valid transforms.

Field evidence (2026-06-29 salmon log, 18:04 / 18:11): ~677k cumulative drops
including sonar_image_down/port/starboard, /tf, cameras, mavros, odom.
Live-display only — recorded survey data is boat-side and complete.

Proposed fix

Make the gate opt-in per topic: a configured allowlist of latest-wins topics
the gate applies to (default empty = gate off for all topics), or equivalently an
exclude-list for sonar/tf/pointcloud. Do not revert — the gate genuinely
fixed the backwards-jumping-stamp complaint on the latest-wins topics.

Stopgap for the next run before this lands: -p drop_stale_packets:=false
(restores pre-change behavior).

Context

• Import PR: Field import: udp_bridge stale-packet gate (2026-06-29) + tests #33 (flags this for a reviewer decision — merge + this follow-up, or
  flip the default within the PR)
• Deployment: Deployment 2026-06-29: Lake Massabesic survey unh_echoboats_project11#356

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Authored-By: Claude Code Agent
Model: Claude Opus 4.8 (1M context)