mxaccesscli: read/write/subscribe System Platform tags via MxAccess

New tool wrapping ArchestrA.MxAccess.LMXProxyServerClass (the same COM
proxy aaObjectViewer / WindowViewer use) as a CliFx CLI for LLM-driven
debugging.

Commands:
- mxa info      — loaded MxAccess assembly identity, supported value
                   types, MxStatusCategory enum.
- mxa read      — fetch one or more tag values; subscribes briefly,
                   captures first OnDataChange per tag, tears down.
- mxa write     — write a value with optional --type coercion; advises
                   first to resolve the attribute type, then waits for
                   OnWriteComplete with a per-call timeout.
- mxa subscribe — stream OnDataChange events for --seconds; JSON Lines
                   under --llm-json for piped agent consumption.
- mxa diag      — minimal smoke test on a private STA thread; bypasses
                   the CliFx pipeline for diagnosing apartment / pump
                   issues.

Implementation notes documented in docs/api-notes.md (reverse-engineered
because AVEVA does not publish a single canonical MxAccess reference):

- Net48 / x86 / [STAThread] are non-negotiable. The CLI runs the entire
  CliFx pipeline on a dedicated STA thread.
- COM events are dispatched as Win32 messages; AutoResetEvent.WaitOne
  alone does not pump them on this configuration. MxSession.WaitForUpdate
  loops Application.DoEvents() + drain + Sleep(20ms) instead.
- Write requires the target attribute's type to be resolved first.
  WriteCommand advises and waits for the initial OnDataChange before
  calling LMXProxyServerClass.Write to avoid ArgumentException
  "Value does not fall within the expected range".
- Errors carry the full MXSTATUS_PROXY[] from MxAccess (Success,
  Category, DetectedBy, Detail) so an agent can tell exactly which
  layer rejected a request.

Verified against the live ZB galaxy with a writeable tag identified
via grdb (TestChildObject.TestInt, mx_attribute_category=10):
  read:      99 (q=192, MxCategoryOk)
  write 7:   round-tripped — read returned 7 — written back to 99
  write str: TestChildObject.TestString round-tripped a timestamp
  subscribe: captured initial value plus subsequent change from a
             separate process

The vendored ArchestrA.MxAccess.dll is gitignored — it is copied from
C:\Program Files (x86)\ArchestrA\Framework\Bin\ on any System Platform
install per the README.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -23,6 +23,7 @@ When in doubt about where content belongs, default to pushing it deeper. `DOCS-G
 - [`graccesscli/`](graccesscli/README.md) — `.NET Framework 4.8 / x86` CliFx-based CLI for automating Galaxy configuration through the ArchestrA GRAccess COM interop.
 - [`grdb/`](grdb/README.md) — SQL/DDL exploration of the Galaxy Repository SQL database (queries, schema, hierarchy/tag-name translation).
 - [`histdb/`](histdb/README.md) — LLM-oriented reference for AVEVA Historian retrieval (extension tables, `wwXxx` time-domain extensions, retrieval modes/options, alarm-event SQL, REST API). Distilled from the official Historian Retrieval Guide.
+- [`mxaccesscli/`](mxaccesscli/README.md) — `.NET Framework 4.8 / x86` CliFx-based CLI for reading, writing, and subscribing to System Platform tags via the **MxAccess** COM proxy (`LMXProxyServerClass`).
 
 ## Tool / resource index
 
@@ -33,6 +34,7 @@ When in doubt about where content belongs, default to pushing it deeper. `DOCS-G
 | Automate Galaxy configuration via GRAccess COM (CLI usage, session daemon, mutations, LLM integration) | [`graccesscli/README.md`](graccesscli/README.md) |
 | Galaxy Repository SQL — connect, schema, hierarchy queries, contained-name ↔ tag-name translation | [`grdb/README.md`](grdb/README.md) |
 | AVEVA Historian retrieval — SQL via `INSQL`, `wwXxx` extensions, retrieval modes/options, alarm/event SQL, REST API | [`histdb/README.md`](histdb/README.md) |
+| Read / write / subscribe to System Platform tags via MxAccess (timeouts, error categories, JSON envelope) | [`mxaccesscli/README.md`](mxaccesscli/README.md) |
 
 ## Maintaining this index