test: add integration suite covering cross-module compositions

[kelsos]

Summary

Populate `packages/plainfp/src/tests/` with the integration suite that the project scope originally planned. Unit + property tests already verify each primitive in isolation; this suite exercises realistic user-facing compositions end to end.

Scenarios

Six files, each stands alone as a readable example of a common usage pattern.

File	What it exercises
`parse-validate-transform.int.test.ts`	fromZod → Result.flatMap → Result.map → match — parse, business, and domain errors short-circuiting at the right layer
`batch-fetch.int.test.ts`	Arrays.map → ResultAsync.fromPromise → ResultAsync.all, plus allWithConcurrency verifying the cap under staggered latencies
`retry-timeout-fetch.int.test.ts`	realistic fetch client: fromPromise + retry + timeout + map + mapError. Recovery from transients, giving up after N, and timeout races
`option-result-interop.int.test.ts`	lookup → toResult → validate → getOr, plus round-trip invariants (toOption ∘ toResult preserves some, discards the error on none)
`data-pipeline.int.test.ts`	the README's headline Arrays.groupBy → Records.mapValues → Arrays.sumBy plus countBy, uniqueBy, and empty-input robustness
`tagged-errors.int.test.ts`	Tagged.tag + hasTag composed with Result + mapError + match for exhaustive discriminated-error handling

Why integration tests

Unit tests structurally can't catch:

• subtle cross-module type weirdness (e.g. Option↔Result interop with narrowing)
• composition ordering bugs (does Result.mapError preserve tag discriminants?)
• real-world usage patterns that look fine per-function but surprise in combination

Each scenario is taken straight from the README or the CONTRIBUTING examples, so these tests also serve as executable documentation.

Results

• 276 total tests passing (was 249; +27 integration cases)
• Typecheck clean
• No runtime code changed — purely new test files
• All test files use realistic domain shapes (users, orders, fetches) rather than foo/bar

Scope doc alignment

Original scope:

Tests colocated (`foo.ts` + `foo.test.ts`); top-level `src/tests/` only for cross-module integration.

`tests/` was created but never populated. This PR fills it.

Test plan

• All required CI checks pass
• Coverage report reflects exercised paths in ResultAsync, Option interop, and Records namespaces
• No CodSpeed regression (these files run as tests, not benches)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.68%. Comparing base (5153437) to head (003916b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   89.16%   91.68%   +2.51%     
==========================================
  Files          23       23              
  Lines         397      397              
  Branches       90       90              
==========================================
+ Hits          354      364      +10     
+ Misses         13       11       -2     
+ Partials       30       22       -8     

Flag	Coverage Δ
unittests	91.68% <ø> (+2.51%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 5 files with indirect coverage changes

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[codspeed-hq]

Merging this PR will not alter performance

✅ 48 untouched benchmarks

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_{Comparing test/integration-suite (003916b) with main (5153437)}