Add the measuringtools plugin (ADR-005) and the plugin-system tutorial

Gage-lab instruments as Asset extensions: measuringtooltypes (color-coded
lookup) + measuringtools (calibration interval/dates, provider, notes) with
calibration status derived at read time (overdue / due soon / current /
unknown), never stored. Full CRUD API with permission-gated writes, types
management with in-use guard, calibration report, nav/reports/config-schema
hooks, and a complete frontend (list/detail/form, types settings page,
calibration report page, gated routes per ADR-009).

First plugin whose migration chain really creates tables post-cutover
(ADR-008), and the working example for docs/PLUGIN-GUIDE.md - a 12-section
walkthrough of building a plugin on this framework, linked from
PLUGIN-QUICKSTART and PLUGINS.

Verified: full suite 323 passing, live E2E on all four pages, fresh
scratch-MySQL migration dry-run green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugins/measuringtools/migrations/script.py.mako

@@ -0,0 +1,24 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision = ${repr(up_revision)}
+down_revision = ${repr(down_revision)}
+branch_labels = ${repr(branch_labels)}
+depends_on = ${repr(depends_on)}
+
+
+def upgrade():
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade():
+    ${downgrades if downgrades else "pass"}