fix(packets): updatePacketsUrl drops ?obs= and ?viewPath= on load and on filter changes

[dborup]

Summary

updatePacketsUrl() in public/packets.js rebuilds the hash from buildPacketsQuery(), which only writes filter params. Every call therefore drops packet-detail params it does not own, obs (selected observation) and viewPath (shareable View Path modal), from #/packets/<hash>?obs=…&viewPath=1. The pane keeps showing the selected observation, but the address bar no longer describes it, so a second refresh or a link copied from the address bar loses it.

#132 stopped the type menu from doing this (it now only refreshes the Clear button via updateClearFiltersVisibility()), but the other call sites remain. Found by the independent re-review of #132.

Where it happens (master 5f493f1d)

• Cold load: init() calls updatePacketsUrl() to show the Clear button when the page loads with URL filters (around line 1984). Opening #/packets/<hash>?obs=123 directly rewrites it to #/packets/<hash> immediately; ?obs was already parsed (around line 1102), so the first render is right, but the URL is not.
• Observer menu and the other filter handlers (region/area onChange, hash/node/channel/filter inputs, time window, sort; around lines 1681–2269).
• viewPath is fork-specific (?viewPath=1, around line 1134); obs also exists upstream (reported there as Kpa-clawbot/CoreScope#2091).

Proposed fix

Make updatePacketsUrl() preserve the detail params it does not own when it rebuilds the query, e.g. read obs and viewPath from the current hash and re-append them after buildPacketsQuery(...). One place, fixes every caller, including init().

Keep buildPacketsQuery() itself filter-only (it is also used for API/other URLs). Consider whether viewPath should survive filter changes or be consumed once (it opens a modal on load); decide and test explicitly.

Acceptance criteria

• Cold load of #/packets/<hash>?obs=123 and …?obs=123&viewPath=1 keeps those params in the address bar.
• Changing observer, region/area, hash/node/channel/filter, time window or sort keeps obs (and viewPath per the decision above) while updating the filter params.
• Clear Filters behaves as today (fix(packets): Clear Filters must reset observer and type selection state #121/fix(packets): empty observer/type selections on Clear Filters #132): filter params removed, detail subpath and params per the same rule.
• Regression tests start from a detail URL with ?obs= (unit + a Playwright check at desktop width); the existing Clear Filters tests keep passing.
• No change to buildPacketsQuery()'s output for callers that use it outside the hash.

Non-goals

No redesign of the filter bar or URL scheme; no deploy.

[dborup]

Fixed by #167 (merged as bedbe1f1).

• updatePacketsUrl() keeps ?obs= (and ?viewPath= while the View Path modal is open) on #/packets/<hash> URLs through cold load, filter changes, Clear Filters, back/forward and new tabs.
• On cold load, obs is read from the hash query; before this, the router dropped it.
• The View Path button now writes viewPath=1.
• Escape closes only the top layer.
• List URLs are written only while the route is #/packets.
• The standalone page writes #/packet/<hash>?obs=….

Leftovers are tracked in #180: modal/overlay behaviour on route change and Escape, a global-search Escape edge case, and Escape on the mobile sheet.