Improve node:buffer slice/subarray shared backing-store parity #1205

Context

The granular node:buffer parity suite covers deterministic slice and subarray range behavior, but it still avoids Node's shared-memory mutation semantics.

Gap

In Node, buf.slice() and buf.subarray() return views over the same backing memory. Mutating the returned view mutates the original buffer, and mutating the original buffer is visible through the view.

Perry currently uses copy-like buffer results for the passing subset.

Desired behavior

• const s = buf.slice(1, 3); s[0] = 0x61; updates buf[1].
• const v = buf.subarray(1, 3); buf[1] = 0x62; updates v[0].
• Existing Node-compatible range handling should be preserved.

Suggested implementation notes

This likely needs a Buffer view representation rather than another copy-only allocation path. Options include extending BufferHeader with backing pointer/offset metadata or introducing a separate view header that all buffer read/write/search methods understand.

Parity tests to add

• test-parity/node-suite/buffer/copy-slice/slice-shared-mutation.ts
• test-parity/node-suite/buffer/copy-slice/subarray-shared-mutation.ts

Validation

• ./run_parity_tests.sh --suite node-suite --module buffer