docs(diagrams): add Mermaid template and architecture metadata headers

[SatoryKono]

Motivation

• Standardize Mermaid .mmd diagram metadata and guidance so subsequent decomposition, styling and CI linting operate from a single canonical template.
• Surface diagram status and approximate node counts in architecture/ diagrams to drive the View-based decomposition and automated SIZE checks.
• Preserve existing theming workflow (external theme/mermaid-config.json + theme/custom.css) and avoid embedding %%{init:} blocks in new files.

Description

• Added docs/02-architecture/mmd-diagrams/_template.mmd containing the canonical metadata block (@version, @date, @type, @level, @status, @view, @parent, @nodes, @adr) and subgraph/style guidance while explicitly advising against %%{init:} blocks.
• Updated all docs/02-architecture/mmd-diagrams/architecture/*.mmd (18 files) to insert %% @status active and %% @nodes ~<count> immediately after the existing %% @level header, preserving existing @version, @date, @type, and @level values.
• Did not modify foundation/ or class-diagrams/ content, and left any legacy %%{init:} blocks intact as per policy; no rendering pipeline changes were made in this PR.

Testing

• Executed the node-count heuristic across the updated architecture/ files using for f in docs/02-architecture/mmd-diagrams/architecture/*.mmd; do count=$(grep -cP '^\s+\w+[\[\(\{>]' "$f" 2>/dev/null || echo 0); echo "$f: ~$count nodes"; done, which produced the approximate counts used for %% @nodes.
• Ran a Python verification script that asserted each modified architecture file contains %% @status and %% @nodes, and the assertions passed for all 18 files.
• Ran repository pre-commit hooks as part of commit validation which failed on unrelated environment/repo checks (missing pip-audit executable and existing root-level VCR cassette guard failures for files: test_chembl_activity_full_run, test_pipeline_idempotency, test_pipeline_resume_after_failure, test_pubchem_compound_pipeline); these failures are unrelated to the documentation-only changes.

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[SatoryKono]

Superseded by #2422.