Closes#1369.
## What
Cross-domain embed support, shipped as two halves:
### Part A — CORS env override + read-only contract
* `applyCORSEnv()` reads `CORS_ALLOWED_ORIGINS` (comma-separated,
trimmed, empties dropped). Set in env → overrides
`cfg.CORSAllowedOrigins`. Unset/empty → config.json value wins.
* `Access-Control-Allow-Methods` tightened from `GET, POST, OPTIONS` →
`GET, HEAD, OPTIONS`. The cross-domain surface is read-only by contract;
same-origin admin writes don't go through preflight and are unaffected.
* `config.example.json` adds `corsAllowedOrigins: []` + a comment
explaining the env override and the embed URL pattern.
* No wildcards introduced (still supported as `["*"]` for ops that opt
in). No credentialed CORS.
### Part B — `?embed=1` chrome suppression
* `shouldEmbedRoute(basePage, hashSearch)` — pure helper, allowlisted to
`map` and `channels`, requires `embed=1` in the hash querystring.
* `navigate()` toggles `body.embed` based on the helper.
* CSS hides `.top-nav`, `[data-bottom-nav]`, `.nav-drawer`,
`.nav-drawer-backdrop`, zeroes body padding/margin, reclaims `100dvh`
for `#app.app-fixed`.
Use: `<iframe src="https://analyzer.example/#/map?embed=1">`. For
iframe-only display, no CORS entry is needed (the iframe loads the
document, not a JSON API). The CORS allowlist only matters when the
embedding origin's own JS calls `/api/*` directly.
## Tests
| File | Asserts | Status |
|---|---|---|
| `cmd/server/cors_embed_1369_test.go` | 4 (env override, env-empty,
env-trim, GET/HEAD contract, preflight POST rejected) | green |
| `test-embed-mode-1369.js` | 9 (helper allowlist + param parsing) |
green |
| `cmd/server/cors_test.go` | existing | updated to read-only method-set
assertion |
TDD: 2 red commits (one per part, both compile, both fail on assertions)
→ 2 green commits.
## Out of scope (per the issue's narrow ask)
* Other SPA routes do not honor `?embed=1` (their chrome makes layout
assumptions; defer until requested).
* No iframe sandboxing recommendation — that's the embedder's
responsibility.
* No CSP / `X-Frame-Options` change in this PR — frames are already
permitted; add an explicit `frame-ancestors` policy in a follow-up if
operators want to whitelist embedders at the HTTP layer too.
## Security notes (DJB lens)
* Allowlist is exact-match, case-sensitive string compare — no
normalization, no scheme/host parsing, no surprises.
* No `Access-Control-Allow-Credentials` (would let third parties read
auth'd state via cookies).
* No reflection of arbitrary origins (every echoed origin came from the
allowlist).
* Methods narrowed to read-only; even a misconfigured allowlist can't
grant cross-origin writes through this middleware.
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