Fix throttle demand buffering and emission scheduling

[Kyle-Ye]

Throttle drops input received without downstream demand and consumes demand before scheduled output runs. With .max(1) and latest: true, a burst of 1, 2, 3 therefore emits 1 instead of Apple's 3.

Use one pending input, a separate scheduling flag, and the next emission time. Consume demand at delivery, schedule buffered input when demand arrives, and select the first input of each window when latest is false. Also match Combine's subscription delivery order, scheduled closure lifetime, completion handling, and buffer cleanup on cancellation.

Add 13 regression tests for demand exhaustion, pending output, window boundaries, reentrant input, optional input, completion, and object lifetime. The state transitions were checked in the local iOS 17 simulator binary, and all regression tests were run against Apple Combine on macOS 26.6.2.

Handle both absolute file URLs and relative paths in SwiftLint baselines, including macOS temporary-directory aliases. This preserves filtering of existing violations with SwiftLint 0.65.0 and 0.65.1.

Validation with Swift 6.3.3:

• Full OpenCombine suite: 1,581 tests, 4 skipped, 0 failures.
• Full Apple Combine compatibility suite: 1,579 tests, 4 skipped, 0 failures.
• All 34 Throttle tests pass against both implementations with -warnings-as-errors.
• 26 additional comparison scenarios produce the same values and emission times as Apple Combine.
• bash utils/lint.sh origin/main: 0 violations with SwiftLint 0.65.0 and 0.65.1, including temporary-directory aliases and paths with spaces.
• Both SwiftLint versions correctly reject an added trailing-semicolon violation.

Fixes #29.

Related to #25.

[codecov]

Codecov Report

❌ Patch coverage is 96.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.32%. Comparing base (710ffc9) to head (aad6ce7).

Files with missing lines	Patch %	Lines
...s/OpenCombine/Publishers/Publishers.Throttle.swift	96.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   96.32%   96.32%   -0.01%     
==========================================
  Files         108      108              
  Lines        8080     8100      +20     
==========================================
+ Hits         7783     7802      +19     
- Misses        297      298       +1     

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[piercifani]

Thanks for merging this! Will a version be tagged?

[Kyle-Ye]

@piercifani done.

See https://github.com/OpenSwiftUIProject/OpenCombine/releases/tag/0.17.0