test(ios-readplace): add a per-file code-coverage gate to make test

[FagnerMartinsBrack]

Stacked on #919 (base = ios/independent-contract-fixes). Review/merge after it.

Purpose

The rest of the monorepo enforces a 100% coverage gate, but the iOS suite ran xcodebuild test with no coverage measurement and no threshold at all — a structural exemption from the repo's coverage doctrine. Nothing stopped iOS logic from landing untested.

This wires a real coverage gate into make test (which the CI ios-tests job runs), so iOS coverage is measured and enforced on every run.

What it solves & the value

• Measures coverage: make test now runs with -enableCodeCoverage YES -resultBundlePath build/TestResults.xcresult.
• Enforces a per-file floor: scripts/check-coverage.py reads the .xcresult and fails the build if any source file drops below its recorded floor. A file with no floor must be 100%, so a newly added logic file cannot land untested.
• Ratchet, not big-bang: the repo's flat 100% isn't met by iOS today (views/OS-glue at 0–5%, logic files at 57–99%), so scripts/coverage-baseline.json seeds each file's floor at today's coverage and excludes pure SwiftUI views and WebKit/OS-boundary glue (the Swift analog of the repo's whole-file OS-boundary exclusion). The later E-series PRs raise these floors toward 100% and move ShareURLExtractor out of the exclusions once it becomes testable.
• No new CI toolchain: the checker uses only the system python3 + xcrun, so the ios-tests job (which only installs xcodegen) needs no extra setup.

Design notes

• Floors carry a deliberate ~1% headroom against minor coverage drift across Xcode versions; the checker only nudges a ratchet once a file sits ≥2% above its floor.
• A Shared file compiles into both the app and extension targets; the checker dedupes by basename and takes the higher coverage.
• A config entry for a file that's no longer measured is a warning, not a hard failure, so config drift never blocks an unrelated PR.

Testing

Full suite green with the gate active locally (207 tests, iOS coverage gate passed: 19 files at or above floor, 10 excluded), plus the STAGING smoke pass. Run as CI runs make test.