Follow-ups to #268, #260 and #212: pktEsc listener leak, test gaps and comment fixes #282

Nits collected from the reviews of #268, #260 (round 2) and #212 (round 2). Details are in the review comments on each PR.

From #260 (#259):

1. I1 — pktEsc leaks a listener. It is added in the packets page init() (public/packets.js ~2363) and has the same leak class as nodesEsc/nodesPanelEsc: a fresh named closure on document per init, never removed. Fix it the same way (at most one listener) and add a test.
2. N1 — a stale comment. The comment above groupIsExpandedInView() (public/packets.js ~2420) still names the bug: /#/packets/<hash> full-page — clicking a different observation doesn't update hex payload or path details Kpa-clawbot/CoreScope#866 deep link as #/packets/<hash>/<obs>. The correct form is #/packets/<hash>?obs=<id>.
3. N2 — an untested guard. The "only while a detail panel is open" guard in _nodesPanelEsc has no test, so mutant ND survives. Add one.

From #268 (#258):
4. A wrong test header. The header comment of test-issue-258-column-widths-e2e.js describes dragging the Path handle left, but the test drags the Time handle right. Fix the comment.
5. A wrong PR claim. The PR text claims that analytics tables "never create an observer". Tables with fewer than 5 usable rows do. Correct it in the docs or code comments where the claim is repeated.

From #212:
6. N1 — an untested transport-route frame. substr(t.raw_hex, 1, 12) in cmd/server/db.go (~3583/3591) is exactly the minimum a transport route needs, but no test sends a transport-route frame through GetChannelMessages. Add one.
7. N2 — two offset sources. The Path Length row in the hex breakdown (public/packets.js ~3875–3897) mixes two offset sources: off from pkt.route_type, and senderPathHashSize(buf). Make them consistent, or add a test that pins that they agree.
8. N3 — exports used only by tests. renderObservedPathHashBadge and OBSERVED_PATH_HASH_TOOLTIP in public/channels.js are only reachable from the test export. Remove them, or document why they are kept.