Add PyBuffer tests, documentation, and examples

- test/py_buffer_SUITE.erl: Common Test suite with 13 tests for:
  - Basic buffer creation (with/without content length)
  - Write/read cycle, readline, readlines
  - Seek/tell position tracking
  - Fast find with memchr/memmem
  - Zero-copy memoryview access
  - Line iteration, closed/empty buffer handling
  - Auto-conversion when passing buffer ref to Python
  - GC and reference counting verification

- docs/buffer.md: Documentation covering:
  - Erlang API (new, write, close)
  - Python API (file-like methods, buffer protocol, find)
  - Architecture diagram and memory layout
  - Performance tips and examples

- examples/py_buffer_example.erl: Working escript demonstrating:
  - Basic buffer usage
  - HTTP body streaming simulation
  - File-like interface methods
  - Zero-copy memoryview access
  - Line iteration for CSV-like data

- CHANGELOG.md: Added PyBuffer API entry under 2.2.0

- docs/getting-started.md: Added Zero-Copy Buffers section with
  link to buffer.md

Author: benoitc

CHANGELOG.md

@@ -4,6 +4,20 @@
 
 ### Added
 
+- **PyBuffer API** - Zero-copy WSGI input buffer for streaming HTTP bodies
+  - `py_buffer:new/0,1` - Create buffer (chunked or with content_length)
+  - `py_buffer:write/2` - Append data, signals waiting Python readers
+  - `py_buffer:close/1` - Signal EOF, wake all readers
+  - Python `PyBuffer` type with file-like interface:
+    - `read(size)`, `readline()`, `readlines()` - Blocking reads with GIL released
+    - `seek(offset, whence)`, `tell()` - Position tracking
+    - `find(sub)` - Fast substring search via memmem/memchr
+    - `memoryview(buf)` - Zero-copy buffer protocol
+    - `for line in buf:` - Line iteration
+  - Auto-conversion: Passing buffer ref to `py:call`/`py:eval` wraps as `PyBuffer`
+  - Suitable for `wsgi.input` in WSGI applications
+  - See [Buffer API docs](docs/buffer.md)
+
 - **Inline Continuation API** - High-performance scheduling without Erlang messaging
   - `erlang.schedule_inline(module, func, args, kwargs)` - Chain Python calls via `enif_schedule_nif()`
   - ~3x faster than `schedule_py` for tight loops (bypasses gen_server messaging)