feat(ablegacy,focas): PLC-5 octal I/O, MicroLogix function files, indirect-address parse, 16i PMC F/G (#244 #245 #247 #265)

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

docs/drivers/FOCAS.md

@@ -91,6 +91,10 @@ on.
 | `MACRO:500` | Custom macro variable | System / user macro variable number |
 | `FIXED:Identity/SeriesNumber` | Fixed-tree field | A path into the [fixed node tree](#fixed-node-tree) — read-only |
 
+PMC letters are series-gated by `FocasCapabilityMatrix.PmcLetters`; the 16i accepts `X Y F G R D` (F/G are
+the CNC-to-PMC / PMC-to-CNC handshake groups its ladders use, #265 — reads only, FOCAS stays read-only), while
+`M C E A K T` remain 0i-F / 30i-family.
+
 The first three name a location the driver asks the CNC for on demand.
 `FIXED:` does not: it names a field of the per-device fixed-tree snapshot the
 driver's own poll loop already maintains, so a `FIXED:` read issues no wire