feat(mcpp.plugins): 0.2.4 — every rule brings its own environment

[Sunrisepeak]

What changes for a consumer

One edge, and no payload list:

[build-dependencies.mcpp]
plugins = { version = "0.2.4", features = ["rules-cuda"], host-module = true }

Each rule declares the xim payloads it drives, under the feature that selects it
and the accelerator it serves. Two gates, and both must open before a byte is
downloaded — a build that names no accelerator installs nothing.

Why the floor moves to 2026.9.6.6 for every member

One change in the engine rather than five in the rules: a payload a dependency
declared is now both installed and answerable. Before it, mcpp::xpkg_dir
compared the whole version position against a directory name, so a rule could
declare >=8.5.0, have it installed, and still be told nothing was there — which
is why every project using a rule repeated that rule's own package list.

The floor is documentation, not a gate: nothing here or in the package's manifest
records a per-package engine floor, and the index-level min_mcpp is
deliberately left alone. What an older client gets is legible at the point of
use, and the descriptor now says so rather than claiming a refusal that does not
exist.

The shape of each default

A judgement about coupling, not a style. An exact version where the payload's
version is coupled to something the rule cannot see — a CUDA runtime must not be
newer than the driver it will meet, and the 12.9 line reaches every driver from
r525 where 13.x raises that to r580. A floor where no such coupling exists:
glslang, dpcpp, the CANN toolkit.

mcpp reads the difference: a bare version is a choice, so a project pinning a
different one wins and the override is reported; a >= is a requirement, so a
project pinning below it is refused naming both sides. Either way one version is
installed.

Verification

• mcpp-plugins CI green on the merge commit 7a834933, including a new step
  that asserts the payloads came from the graph pass rather than the root's:
  no [xlings.workspace] in any device fixture, and mcpp's own
  entries declared by dependencies line for each.
• Both mirrors downloaded and compared byte for byte; one sha256 names both
  (abcf165e…).
• The descriptor parses under lua, and latest is 0.2.4 on all three
  platform tables.