…nd regions

Global buffer pool for octet buffers with three size classes:
  Small (16B, max 64 free), Medium (1KB, max 32), Large (16KB, max 32).

Lock-free via CAS (Treiber stack): sb-ext:cas on SBCL,
sys:compare-and-swap on LW, generic lock fallback.

allocate-buffer returns fill-pointer arrays with fp = requested size.
(length buffer) returns the requested size; array-total-size returns
the size-class capacity.  deallocate-buffer resets fp before pooling.

with-resource-usage-region: scope-based deallocation for buffers that
escape local management (e.g., HEADERS continuation closures).
region-track-buffer transfers ownership to the region.

Measured on SBCL (50 H2 GET + 10 POST, Apple M3):
  Small:  2 allocated, 2402 recycled (1201:1)
  Medium: 5 allocated, 1419 recycled (284:1)
  Large:  1 allocated, 9 recycled (9:1)

- Add to .asd and package.lisp
- Move documentation to mgl-pax.
- Align file and package name
- Use buffer-pool package from core

HTTP-STREAM-ERROR now derives from ERROR. There is a specific subclass to be
raised when we receive the stream error from the peer, as opposed from when we
detect it.

New generic function QUEUE-FRAME-REGION to write only part of the provided data.
Falls back to QUEUE-FRAME.

WRITE-VECTOR-FRAME can be now used to send prepared data vector without copying
it, providing QUEUE-FRAME-REGION is specialized for the connection to avoid
copying.

QUEUE-FRAME-REGION uses WRITE-VECTOR-FRAME now.

There were two functions with same interpretation and it did not work.

Relocate the method, scheme, authority, path slots from server-stream
up to the http2-stream common superclass.  Both server-stream and
client-stream instances now inherit the slots (and their accessor
generic functions).

Rationale: log-closed-stream in core/frames/headers.lisp calls
(get-path stream), (get-method stream) etc. on any h2-stream during
http-stream-error handling.  The slots were only on server-stream,
so these calls signalled no-applicable-method-error on client
streams, killing client reactors.

Moving to http2-stream keeps the existing server-stream API (all
slots still accessible via the same accessor GFs) while making the
accessors work uniformly on client streams for diagnostics and
debugging.  Backward-compatible: server-stream instances retain
all slots via inheritance, and no existing code paths change.

…aders

Add request pseudo-header slots to client-stream

Should fix LOG-CLOSED-STREAM on client streams (presently raises error)

The mode line attribute said Package: http2/resources but the
in-package form uses http2/buffer-pool.  Genera reads the mode
line to set the package, so the mismatch causes compilation in
the wrong package.

FIND-JUST-STREAM-BY-ID returns the keyword :CLOSED when the
stream-id is absent from the streams table (the stream was already
closed and reaped).  APPLY-STREAM-PRIORITY then ran the default
method, which calls (SETF (GET-WEIGHT stream) WEIGHT) and (SETF
(GET-DEPENDS-ON stream) ...) -- neither has a method specialised
on a keyword.

Symptom in production: a PRIORITY-bearing frame (or a HEADERS
frame with the priority flag) for a closed stream made the H2
writer thread die with

  SIMPLE-ERROR -- No applicable methods for
  #<STANDARD-GENERIC-FUNCTION (SETF GET-WEIGHT) ...>
  with args (WEIGHT :CLOSED)

RFC 9113 sec 5.5 explicitly permits PRIORITY frames on closed
streams, and RFC 9218 deprecates stream priority entirely, so the
right behaviour is "do nothing" -- there is no per-stream state
to update.

The library already follows this pattern for the three other
generics that FIND-JUST-STREAM-BY-ID's keyword return value
reaches:

  - update-window-size      (eql :closed)
  - peer-resets-stream      (eql :closed)
  - get-peer-window-size    (eql :closed)

apply-stream-priority was missing one.  Adding it fixes the
crash without changing behaviour on live streams.