PL/Python-style $_SD and native bytea transform (bytea_plphp)

[jdatcmd]

Closes the two PL/Python-parity gaps PL/php had. The session-global GD
equivalent already exists as $_SHARED; this adds the per-function private
dictionary and the native bytea mapping.

$_SD (per-function private data)

Each function now has an $_SD associative array that persists across calls to
that function within a session and is private to it, the per-function
counterpart to session-global $_SHARED. The pair mirrors PL/Python's SD/GD.

Implemented by prepending static $_SD = array(); to every generated wrapper.
The wrapped function is compiled once per session, so the static persists across
calls; a CREATE OR REPLACE installs a fresh op_array, so $_SD resets. No
per-call C lifecycle, no struct changes.

sql/sd.sql covers persistence across calls, per-function isolation, $_SD vs
$_SHARED, reset-on-redefine, structured values, and use inside a trigger.

bytea_plphp (native bytea transform)

A new transform extension mapping bytea to a raw, binary-safe PHP string and
back, mirroring PL/Python's bytea <-> bytes. The default text path routes
bytea through byteaout/byteain (a \x... hex string) and truncates on
embedded NULs; the transform passes the bytes themselves.

Modelled on jsonb_plphp, so it reuses the existing transform machinery and
also applies in nested contexts (a bytea column in a composite result, etc.).
Non-string returns are rejected with a clear error. varatt.h is included only
on PG 16+, where the varlena macros moved out of postgres.h.

Docs / CI

Language reference ($_SD section, bytea_plphp transform section), PL/Perl and
PL/Tcl comparison tables, README, and CHANGELOG ([Unreleased]) updated.
bytea_plphp is built and tested in both the CI matrix and the ASAN job,
alongside jsonb_plphp.

Testing

Verified in the plphp-build container on PG 18 / PHP 8.3: all 26 core tests
(including the new sd) plus the jsonb, hstore, and bytea suites pass.
The full PG 11-18 matrix runs here via CI.

🤖 Generated with Claude Code