tests: extract URL helper for sub-path-safe page.goto across the suite

[rdhyee]

Summary

Adds tests/playwright/helpers/url.js exposing siteUrl(path, suffix?) and explorerUrl(suffix?). Migrates all 8 specs in tests/playwright/ to use them, replacing the hand-rolled BASE_URL / EXPLORER_PATH constants and the two patterns that previously coexisted.

Why

When verifying PR #237 against the rdhyee fork staging URL (https://rdhyee.github.io/isamplesorg.github.io/), tests/playwright/facet-viewport.spec.js silently 404'd. Root cause: EXPLORER_PATH = '/explorer.html' (leading slash) called via page.goto(\${EXPLORER_PATH}${HASH}`). Playwright resolves an absolute path against the **origin** of baseURL, dropping the /isamplesorg.github.io` sub-path segment.

Inventory of the suite revealed two coexisting URL-construction patterns, each with a latent bug exposed only on sub-path deploys:

Pattern	Specs	Bug
page.goto(\${BASE_URL}${EXPLORER_PATH}...`)` (4 specs)	cesium-queries, explorer-helper, explorer-layout-stability, explorer-map-overlay	Requires TEST_URL to NOT have a trailing slash; a trailing slash produces //explorer.html
page.goto('/explorer.html...') relying on Playwright baseURL (4 specs)	facet-viewport, facetnote-url-load, search-real-count, url-roundtrip	Drops sub-path on TEST_URLs like https://rdhyee.github.io/isamplesorg.github.io/

Rather than fix each spec independently (Codex round-1 review's recommendation on the earlier single-spec patch), this PR extracts the URL construction into one helper.

The helper

const BASE_URL = (process.env.TEST_URL || 'http://localhost:5860').replace(/\/$/, '');

function siteUrl(path, suffix = '') {
    return `${BASE_URL}${path}${suffix}`;
}

function explorerUrl(suffix = '') {
    return siteUrl('/explorer.html', suffix);
}

The trailing-slash strip means the helper produces the same URL whether TEST_URL ends in / or not — eliminating a foot-gun any operator running the suite against staging would otherwise hit.

Migration

All 8 specs replaced local BASE_URL/EXPLORER_PATH constants with const { explorerUrl } = require('./helpers/url') (or siteUrl for cesium-queries.spec.js, which navigates /tutorials/parquet_cesium.html).

Diff is mostly mechanical:

// Before (Pattern 1)
const BASE_URL = process.env.TEST_URL || 'http://localhost:5860';
const EXPLORER_PATH = '/explorer.html';
await page.goto(`${BASE_URL}${EXPLORER_PATH}#v=1&...`, { ... });

// Before (Pattern 2, broken on sub-path)
const EXPLORER_PATH = '/explorer.html';
await page.goto(`${EXPLORER_PATH}#v=1&...`);

// After
const { explorerUrl } = require('./helpers/url');
await page.goto(explorerUrl('#v=1&...'), { ... });

Test plan

• facet-viewport.spec.js passes 4/4 on localhost in isolation — same as pre-PR baseline (verified before and after migration)
• Other migrated specs have pre-existing failures unrelated to this PR. Proven by stashing my changes and re-running explorer-helper.spec.js against the pre-migration version — same 2/4 failures appear. Stash restored. These pre-existing failures are tracked separately from this PR.
• Tests now reach the explorer on sub-path deploys like the rdhyee fork staging URL (verified via direct probe of the bbox-shrink behavior on staging — Cyprus at 500km alt returns 96,699 samples vs 5,980,282 global, matching raw DuckDB bbox queries)
• CI smoke gate stays green — the smoke gate uses localhost, which the helper handles identically to the old code (string concat, no trailing slash to strip)

Out of scope (filed separately or noted)

• The pre-existing failures in explorer-helper.spec.js, explorer-layout-stability.spec.js, explorer-map-overlay.spec.js, and the 14 cesium-queries.spec.js tests (the latter all 404 because docs/tutorials/parquet_cesium.html isn't rendered) are not addressed by this PR.
• The 2 timing-sensitive facet-viewport failures against rdhyee staging (heavy bbox JOIN over Fastly→GH-Pages→remote R2 parquet) are environmental fragility, not a regression in PR explorer: B1 viewport-aware facet counts (#234 step 3) #237. To be tracked separately if recurring against production matters for sign-off.

Context

Discovered 2026-05-27 verifying PR #237 against the rdhyee fork staging deploy. The original commit on this branch fixed only facet-viewport.spec.js; Codex's round-1 review recommended extracting the shared helper rather than duplicating the fix into every spec. This PR adopts that recommendation.