test: cmark memory/leak/concurrency/multibyte-position safety harness

[hebertialmeida]

Adds a test-and-tooling-only safety net around the cmark_gfm wrapper, with no production code changes. This hardens coverage of the failure modes that the current suite is structurally blind to, so a future performance change to CMNode (reducing wrapper-object allocation) can be made safely.

What's added

• MultibytePositionTests — byte-vs-character position correctness (emoji/CJK/combining/ZWJ/CRLF/tabs), asserted at the substring level.
• MemorySafetyTests — retained nodes outliving the document, Iterator reset/re-enumerate, tight re-parse and wrapper-graph churn loops.
• LeakTests — retain-cycle weak-ref checks + bounded-growth loops.
• ConcurrencyTests — hundreds of concurrent parses (same/different inputs).
• CI — Address- and Thread-sanitizer test steps.

Results

• 62 → 98 tests (+36). Green under plain swift test, ASan, and TSan — zero sanitizer diagnostics.

Findings

• No latent memory-safety, leak, concurrency, or multibyte-position bug. The utf8.index-based position mapping is already byte-correct; the old tests only used ASCII (input.range(0...N)), where char- and byte-offsets coincide, so multibyte was untested — now closed.
• Pre-existing, not introduced here: ParserInlineTests.testInvalidLink crashes locally (index-out-of-range on an unexpected parse shape) but is gated by XCTSkipIf(isCI), so CI stays green; it awaits an upstream swift-cmark fix.

Purpose

Prerequisite for a later, guarded CMNode wrapper-allocation reduction: the three ownership-break failure modes are now watched by sanitizers on every PR (a dropped owner → ASan use-after-free; a leaked node/cycle → LeakTests + ASan; shared mutable wrapper state → TSan race). Not covered: allocation/perf baselines (safety only, not churn magnitude).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8cb12b3) to head (dc00264).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          137       133    -4     
=========================================
- Hits           137       133    -4     

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