manifest: component pins are not layer content

A component pin bump counted as a platform change: the layer content hash
in the platform identity covered the recipe carrying the SRCREV, the
platform is folded into every acceptance fingerprint, so every image
that carried any component update invalidated the whole catalog (dev
image 20260816191951: every test domain-changed after a one-line
forgectrl bump; the two manifests differ only in
platform.layers.meta-forgefirm). The component entry already identifies
the pinned source file by file; the pin double-counted it.

Component pins now live in <recipe>-pin.inc (SRCREV and the PV that
moves with it, nothing else) - forgectrl, grblhal-glowforge and
forgefirm-app here, the BSP components in meta-openglow - and
forgefirm-image-manifest.bbclass leaves *-pin.inc out of the layer
content (FORGEFIRM_MANIFEST_PIN_SUFFIX). Recipe bodies, patches, config
fragments, init scripts and third-party pins with no manifest entry stay
layer content; a pin written into a recipe body still hashes (the safe
direction). manifest-from-tree.py mirrors the rule and reads pins
through the recipe's requires; test_tree_manifest.py proves both
(pin bump: hash unchanged; recipe body or inline pin: changed).
Bitbake resolves the same SRCREV/PV for every pinned recipe.

Docs: ACCEPTANCE.md (what layer content is), kas/README.md (the pin
files in the push order), BRINGUP.md (the finding and the bench
consequence: the first image built with the pin files is itself a
platform change, so its campaign is a full one; pin bumps inherit
after it).

No catalog consequence: nothing in the image's behavior changes; the
change is to the acceptance identity computation, proven by the unit
tests and the CI lint on the tree manifest.

docs/ACCEPTANCE.md

@@ -53,7 +53,16 @@ Consequences:
   A panel-only change reruns the core plus the panel tests, not the
   cooling drills.
 - A platform change (kernel, device tree, a layer's content) invalidates
-  everything.
+  everything. Layer content is every file under `meta-forgefirm`,
+  `meta-glowforge-bsp` and `meta-openglow-core` except documentation
+  (`*.md`) and the component pin files (`<recipe>-pin.inc`, holding only a
+  component's `SRCREV` and the `PV` that moves with it). A pin bump is the
+  component's change, and the component entry already carries it file by
+  file, so it invalidates the tests that cover the component - not the
+  bench. A recipe-body change (build flags, patches, config fragments,
+  init scripts, a third-party pin with no manifest entry) is layer content
+  and invalidates everything; so does a pin written into a recipe body
+  instead of its pin file (the safe direction).
 - A change to a test's implementation invalidates that test's earlier
   passes and no other.
 - "Touched" is computed from content hashes carried in the image, never