Add structured GPIB command reference (commands.json) and docs

Complete extraction of all 97 commands in HP 3457A Operating.pdf
Chapter 4, expanded to 120 JSON entries (F10-F58 modeled as 24
independent commands rather than one parameterized command). Each
entry includes description, syntax, structured parameters, important
points, worked examples, and tier/rationale classification to guide
which commands a Python control library actually needs.

Documents several findings not obvious from the manual alone: the
output buffer's single-reading data-loss behavior and the correct
burst-acquisition pattern, the CR/LF bus-hold gotcha, the Front Panel
checkbox's unreliability, the isolation-link mechanism behind F10-F58's
speed, and memory/timing capacity math for burst captures.

Updates README.md (was empty) and cmdformat.txt to document the
schema and key findings.

cmdformat.txt

@@ -1,15 +1,53 @@
-command name
-local y/n
-remote y/n
-description
-syntax
-parameters
->> parameter name
->> parameter description
->> parameter values
->>>> value description
->>>> power-on value y/n
->>>> default value y/n
-Important points
-example
+Schema for entries in commands.json (the "commands" array).
+Read commands.json's own "_meta" block for the full rationale behind each
+field below and for cross-cutting findings that don't belong to any one
+command.
 
+command name                   -> "name"
+manual page(s)                 -> "manual_pages" (array of printed page numbers, e.g. ["4-7","4-8"])
+local y/n                      -> "local" (Front Panel checkbox; see _meta.front_panel_caveat before trusting this)
+remote y/n                     -> "remote" (Remote checkbox; only ADDRESS is false)
+description                    -> "description"
+syntax                         -> "syntax" (null for the four service-only commands)
+parameters                     -> "parameters" (array, empty [] if the command takes none)
+>> parameter name              -> parameters[i].name
+>> parameter description       -> parameters[i].description
+>> parameter values             -> parameters[i].values (array of choice/range rows; empty [] for free-form numeric parameters)
+>>>> value description          -> values[j].description
+>>>> power-on value y/n         -> values[j].power_on (bool, only present for discrete enumerated choices)
+>>>> default value y/n          -> values[j].default (bool, only present for discrete enumerated choices)
+>> (free-form parameters instead carry parameters[i].power_on_raw / .default_raw as plain text)
+Important points               -> "important_points" (array of strings)
+example                        -> "examples" (array of {code, comment, note?})
+
+Fields added beyond the original sketch, once the actual manual data made
+clear they were needed:
+
+tier                           -> "core" | "protocol" | "extended" -- does a typical
+                                   automated workflow need this command? See
+                                   _meta.tiering.
+tier_reason                    -> free text explaining the tier call for this
+                                   specific command.
+rationale                      -> "measurement_hardware" | "acquisition_timing" |
+                                   "wire_efficiency" | "era_computer_workaround" |
+                                   "plugin_card_dependent" | "diagnostic_calibration" |
+                                   "status_error_readback" | "ui_frontpanel_only" |
+                                   "bus_protocol" -- WHY the command exists / is fast,
+                                   independent of whether a library needs it. See
+                                   _meta.rationale_taxonomy.
+service_command                -> true for the 4 commands (CAL, DIAGNOSTIC, SECURE,
+                                   REV?) the manual defers entirely to the Service
+                                   manual, with no documented syntax here.
+see_also                       -> present on R and T, the bare-syntax aliases of
+                                   RANGE and TRIG -- points to the real command
+                                   instead of duplicating its content.
+family                         -> "F10-F58" on each of that family's 24 entries,
+                                   linking them together (see _meta.f10_f58_split
+                                   for why they're split into 24 entries instead of
+                                   one parameterized command).
+selects_function / selects_range -> present only on the F10-F58 family entries;
+                                   what that specific zero-parameter command
+                                   configures.
+prerequisites                  -> present only on "?" (the standalone trigger
+                                   shorthand); the TARM/NRDGS/TRIG state it requires
+                                   before it will work.