feat: add suitelink client runtime and test harness

tests/SuiteLink.Client.Tests/Fixtures/FixtureBytes.cs

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+namespace SuiteLink.Client.Tests.Fixtures;
+
+internal static class FixtureBytes
+{
+    public static byte[] Read(string fileName)
+    {
+        var path = Path.Combine(AppContext.BaseDirectory, "Fixtures", fileName);
+        return File.ReadAllBytes(path);
+    }
+}

tests/SuiteLink.Client.Tests/Fixtures/README.md

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+# SuiteLink Test Fixtures
+
+This folder stores small binary packet fixtures used by protocol codec tests.
+
+## Source and intent
+
+- These fixtures are based on fixed vectors already used in tests and aligned with the reverse-engineered SuiteLink dissector behavior used in this repo's design docs.
+- They are intentionally minimal and validate specific assumptions, not full protocol coverage.
+
+## Fixtures
+
+- `handshake-ack-normal.bin`
+  - Bytes: `06 00 01 00 A1 B2 C3 A5`
+  - Assumption validated: normal handshake ACK frame type is `0x0001` with payload bytes preserved.
+- `advise-tagid-11223344-item-A.bin`
+  - Bytes: `0A 00 10 80 44 33 22 11 01 41 00 A5`
+  - Assumption validated: ADVISE message type bytes are `10 80` (little-endian `0x8010`) and item encoding is UTF-16LE length-prefixed.
+- `update-binary-tag-1234-true.bin`
+  - Bytes: `0D 00 09 00 34 12 00 00 0A 00 C0 00 01 01 A5`
+  - Assumption validated: UPDATE type bytes are `09 00` and binary value decode path maps to `true`.
+
+## Notes
+
+- Keep fixture count modest and focused on wire-level assumptions that are easy to regress.
+- If a fixture changes, update corresponding tests and document why the protocol assumption changed.