Restore 100% coverage under Vitest 4 (5.2.0)

[terehov]

Follow-up to #370 before the 5.2.0 release. The Vitest 4 bump left coverage at 99.95 / 98.71 / 99.68 while CI stayed green: the new AST-based remapper counts the implicit else of every if, reports uncalled arrow functions, and binds v8 ignore hints to a single AST node (so next 3 counts stopped working), and neither Codecov nor vitest enforced a floor.

Coverage

• Back to 100% on statements, branches, functions and lines: 34 new behavior tests (prototype-pollution guards in both spread orders, EPIPE on stdout, stale file-stream errors, malformed/hostile source maps, partial req/res shapes, unknown worker messages, multi-write flush, production-silenced diagnostics, ...), a handful of provably redundant guards simplified, and justified v8 ignore else hints only for defensive paths no caller can reach.
• coverage.thresholds in vitest.config.ts now pins all four metrics at 100%, so npm run coverage (the Node 20 CI job) fails on the next drop.

Fixed (found while covering those paths, all in the 5.2.0 changelog)

• A frozen or getter-based default LogObj made every log call throw Cannot assign to read only property.
• A source map that is valid JSON but structurally wrong ("sections":[null], non-string mappings) threw a TypeError out of the log call.
• Worker transport flush() rejected forever after a failed new Worker() although writes had already gone inline.
• restoreConsole() left tslog's forwarder on a console method that did not exist before wrapConsole().

Verified locally: Node 24 (1675 tests, 100% coverage), Bun, Chromium browser suite, tsgo, biome, doc-sync.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7b7085f) to head (3d91991).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #373   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           53        53           
  Lines         4642      4628   -14     
  Branches      1425      1361   -64     
=========================================
- Hits          4642      4628   -14     

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