test: property-based tests for Result/Option/Arrays/pipe laws

[kelsos]

Summary

Batch #4 of the post-launch polish. Add fast-check properties that verify the algebraic laws the library implicitly claims. Each property runs ~100 random cases by default, so this PR adds roughly 2,500 automated check cases on top of the existing example-based tests — cheap insurance against entire classes of regressions.

What gets verified

Result (`result/laws.prop.test.ts`)

• Functor identity: `map(r, id) === r`
• Functor composition: `map(map(r, f), g) === map(r, g ∘ f)`
• Monad left identity: `flatMap(ok(x), f) === f(x)`
• Monad right identity: `flatMap(r, ok) === r`
• Monad associativity: `flatMap(flatMap(r, f), g) === flatMap(r, x => flatMap(f(x), g))`

Option (`option/laws.prop.test.ts`)

Same five laws with `flatMap` + `some`.

Arrays (`arrays/transform.prop.test.ts`)

• Functor identity + composition for `map`
• `filter` length never exceeds input
• `filter` idempotence — applying the same predicate twice is the same as once
• Identity and absorption: `filter(true) === xs`, `filter(false) === []`
• `flatMap` singleton lift: `flatMap(xs, x => [x]) === xs`
• `flatMap` annihilation: `flatMap(xs, () => []) === []`
• `reduce(xs, 0, +)` agrees with native `xs.reduce`

pipe + flow (`pipe.prop.test.ts`)

• Identity: `pipe(x) === x`
• Single-arg application: `pipe(x, f) === f(x)`
• Two-step composition: `pipe(x, f, g) === g(f(x))`
• Associativity: `pipe(pipe(x, f), g) === pipe(x, f, g)`
• flow/pipe equivalence: `flow(f, g)(x) === pipe(x, f, g)`
• `flow(id)` is the identity function

Implementation

• `fast-check@4.6.0` added as a plainfp devDep (41 days old, past the 7-day minimumReleaseAge wall; no allowlist entry needed).
• Tests named `.prop.test.ts` — still picked up by the existing `src/**/.test.ts` vitest include pattern.
• 25 new test blocks, ~2,500 generated cases per run.

Results

• 249 total tests passing (was 224)
• Typecheck clean
• No runtime code touched

Test plan

• All required CI checks pass
• Coverage job stays within threshold (property tests exercise code paths too, so coverage should tick up)
• Test job reports the new files in its run output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.16%. Comparing base (a4b6d4f) to head (5153437).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   88.41%   89.16%   +0.75%     
==========================================
  Files          23       23              
  Lines         397      397              
  Branches       90       90              
==========================================
+ Hits          351      354       +3     
+ Misses         15       13       -2     
+ Partials       31       30       -1     

Flag	Coverage Δ
unittests	89.16% <ø> (+0.75%)	⬆️

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see 1 file 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/property-based (5153437) with main (a4b6d4f)}