test(build): make the fake fetch method-aware and header-readable

[netravnen]

The node:test harness could not express the cases needed to test
lib/admincom-common.js's retry/backoff wrappers, which is why that code
shipped with a defect nothing caught.

Two limits caused it. The fake response exposed only headers.forEach(),
while fetchWithRetry reads response.headers.get('retry-after') -- so any
503 fixture threw TypeError before reaching the logic under test. And the
fake fetch was async (url) =>, dropping the init argument entirely, so a
test could not observe the request method or count requests. Since a
retried request and a single one return the same value, request count was
the only observable that distinguishes them, and it was unavailable.

This closes both so the following commit can test the retry policy, and
pins the harness's own capabilities: a gap in the harness is invisible in
a green run, and produces confident, empty coverage.

Changes:

• Extracted makeFakeResponse(status, body, headers) with a real
  case-insensitive headers.get()/has()/forEach(), correct ok derivation,
  and json()/text().
• makeFakeFetch now takes (url, init) and appends { url, method, body,
  headers } to a per-load call log, returned from loadScript as
  fetchCalls.
• fetchMap entries may now be a response descriptor
  ({ __response: true, status, body, headers }) to model failures and
  header-bearing responses; a bare JSON body still means 200, unchanged.
• Added tests/browser-shim.test.js asserting these capabilities directly.

Security:

• N/A -- test harness only, never inlined into a generated .user.js.

Testing:

• node --test from user.js/: 480 tests, 479 pass, 1 skipped (live tests
  are opt-in), up from 476/475.
• All 476 pre-existing tests pass unchanged, confirming the bare-body
  fetchMap form and the 404-on-unmapped-URL behavior are preserved.

Backwards Compatibility:

• Purely additive for existing callers: the descriptor form is opt-in via
  an explicit __response marker, and loadScript's return value gains a
  property rather than changing one.

Assisted-by: Claude:claude-opus-5

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