fix: do not let --all overwrite coverage of files that were loaded

[Eljees]

Fixes #1372

--all can wipe out the coverage of files that were exercised, which is why the report comes back at
0% for everything rather than just for the untouched files.

addAllFiles() writes its placeholder straight into the global coverage object, unconditionally:

const coverage = coverageFinder()
const lastCoverage = this.instrumenter().lastFileCoverage()
if (lastCoverage) {
  coverage[lastCoverage.path] = {
    ...lastCoverage,
    // Only use this data if we don't have it without `all: true`
    all: true
  }
}

The comment already states the intended rule — only use this data if we don't have it without
all: true — but there is no condition in the code implementing it. When a file already has real
counters in coverage[path], the freshly instrumented (all-zero) copy replaces them.

The fix is the missing check. Everything else stays as it was: files that were never loaded still get
their zeroed placeholder.

Before / after

New case in test/add-all-files.js — real counters are put in the global coverage object first, then
addAllFiles() runs, and the test asserts they survived:

before:  not ok - real hit count survives --all
         not ok - entry is not marked as an --all placeholder
         # { total: 15, pass: 13, fail: 2 }

after:   ok - real hit count survives --all
         ok - entry is not marked as an --all placeholder
         # { total: 15, pass: 15 }

Run with tap test/add-all-files.js after npm run instrument, on node 24.

I kept the wording of the original comment and only moved it above the if, since it now describes a
condition that actually exists.

AI-assisted (LLM used for drafting); the change and both runs above are mine.