buffer: support aligned allocations

[ronag]

Adds an optional alignment argument to Buffer.allocUnsafe() and
Buffer.allocUnsafeSlow(), guaranteeing that the memory backing the returned
buffer starts at an address that is a multiple of alignment.

// Block-aligned buffer, suitable for a read on an O_DIRECT descriptor.
const buf = Buffer.allocUnsafeSlow(4096, 4096);

Why

Some operating system interfaces refuse to work with unaligned memory. The
motivating case is unbuffered ("direct") file I/O: a read or write on a
descriptor opened with O_DIRECT fails with EINVAL unless the buffer address
is a multiple of the logical block size of the underlying device. There is
currently no way to obtain such a buffer from JS, since the address of a backing
store can neither be observed nor chosen.

Alignment is also worth having purely for performance, for instance to keep a hot
buffer from straddling one more cache line than its size requires.

How

V8 does not allow picking the address of a backing store, so alignment is
achieved by over-allocating alignment - 1 bytes and positioning the buffer at
the first suitably aligned byte within them. A new arrayBufferAlignedOffset()
binding computes that offset.

It returns an offset rather than a finished Buffer for two reasons: external
backing stores are rejected outright when the V8 sandbox is enabled, and
Buffer::New() is not usable during buffer.js evaluation — the buffer
prototype is only wired up after require('buffer') returns, which the pool
creation runs before.

Consequently buf.byteOffset is usually non-zero and buf.buffer is larger than
size, as is already the case for pooled buffers. This is documented.

The Buffer.allocUnsafe() pool is now cache-line aligned itself, so pooled
allocations can satisfy any alignment up to 64 bytes by padding their offset into
the pool rather than allocating separately. poolOffset is therefore tracked
relative to a new poolBase.

alignment must be a power of two no larger than 2 ** 30, and
size + alignment - 1 must fit within buffer.constants.MAX_LENGTH.

Notes for reviewers

• The alignment of the resulting address is not observable from JS, so the
  binding CHECKs it; the added test sweeps sizes × alignments over both APIs to
  exercise those assertions.

[nodejs-github-bot]

Review requested:

• @nodejs/userland-migrations

[Copilot]

Pull request overview

This PR adds optional alignment support to Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() so callers can request that the returned Buffer’s backing memory starts at an address aligned to a power-of-two boundary, enabling use cases like O_DIRECT I/O and some alignment-based performance optimizations.

Changes:

• Add a new internal binding (arrayBufferAlignedOffset) and supporting C++ allocation helper to compute aligned view offsets into over-allocated ArrayBuffers.
• Implement aligned allocation plumbing in lib/buffer.js (including cache-line-aligned pool base and aligned pooled slicing up to 64 bytes).
• Add documentation and tests covering aligned allocation behavior and argument validation.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/parallel/test-buffer-alloc-alignment.js	Adds test coverage for aligned allocations and validation behavior.
src/node_buffer.cc	Introduces AllocateUnsafeArrayBuffer() and the arrayBufferAlignedOffset() binding.
lib/internal/buffer.js	Adds createUnsafeAlignedBuffer() used by Buffer APIs and pool creation.
lib/buffer.js	Adds alignment parameter handling, alignment validation, and aligns the pool base.
doc/api/worker_threads.md	Updates Buffer.allocUnsafe() documentation link anchor.
doc/api/deprecations.md	Updates Buffer.allocUnsafe/allocUnsafeSlow documentation link anchors.
doc/api/buffer.md	Documents the new alignment argument and adds an “Aligned allocations” section.

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[codecov]

Codecov Report

❌ Patch coverage is 87.20000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (f00fb75) to head (ef04e22).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/node_buffer.cc	65.62%	2 Missing and 9 partials ⚠️
lib/internal/buffer.js	82.35%	3 Missing ⚠️
lib/buffer.js	97.36%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #65003    +/-   ##
========================================
  Coverage   90.27%   90.27%            
========================================
  Files         762      762            
  Lines      247534   247634   +100     
  Branches    46694    46715    +21     
========================================
+ Hits       223457   223553    +96     
+ Misses      15529    15498    -31     
- Partials     8548     8583    +35     

Files with missing lines	Coverage Δ
lib/buffer.js	99.02% <97.36%> (-0.10%)	⬇️
lib/internal/buffer.js	98.50% <82.35%> (-0.25%)	⬇️
src/node_buffer.cc	69.27% <65.62%> (-0.13%)	⬇️

... and 37 files with indirect coverage changes

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