fix: throw when a RingBuffer is modified during enumeration [patch]

[matt-edmondson]

Summary

After #70 / #84, RingBuffer<T> was the only container whose enumerator did not fail fast. On a full buffer, calling PushBack inside a foreach shifts every logical index, so the loop silently skipped an element. Calling Resample or Resize mid-loop also corrupted the enumeration without raising an error.

This follows the ContiguousCollection pattern:

• a private version field, incremented in PushBack, Clear and AllocateBuffer. AllocateBuffer covers both Resize and Resample.
• GetEnumerator() now returns Enumerate(version), which calls Enumeration.ThrowIfModified before each yield and once after the loop.

Tests

• New tests check that a modification during foreach throws InvalidOperationException for each mutator: PushBack, Clear, Resize and Resample.
• Another test covers a modification after the last element, which throws on the final MoveNext.
• A control test confirms that modifying the buffer between two enumerations does not throw.
• I ran the five throwing tests against the unfixed code first and all five failed. They pass with the change.
• Full suite: 429/429 pass locally (net10.0).

This PR is independent of #99 (#85, eviction leak). Both touch PushBack but in different lines, so they merge cleanly in either order.

Fixes #86

🤖 Generated with Claude Code

https://claude.ai/code/session_01UoSUYMQMykgfsb5y6AhnKb

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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