Add USER-level SysVar.setString / SysVar.createString JSON-RPC methods

[claymore666]

What

Adds two JSON-RPC methods at LEVEL=USER, completing the typed system-variable API for the string type:

• SysVar.createString — {name, init_val, internal, chnID}
• SysVar.setString — {name, value}

Closes #129.

Why

string is a first-class system-variable type — SysVar.getAll reports it as STRING, and the WebUI's own editor (rega/pages/tabs/admin/msg/newSysVar.htm) offers it as one of five creatable types. But unlike bool, float, and enum — each of which has SysVar.create* and SysVar.set* at LEVEL=USER — string had no typed method at all.

The only JSON-RPC path to create or write a string sysvar was therefore ReGa.runScript, which is LEVEL=ADMIN. As a result a normal (USER) account can fully manage bool/float/enum sysvars but cannot create or set a string one without full admin rights. This change removes that asymmetry.

How

The two methods mirror the existing enum/float implementations:

• createstring.tcl mirrors createenum.tcl, but sets sv.ValueType(ivtString) and sv.ValueSubType(istChar8859) — the same incantation the WebUI uses in rega/esp/system.fn, so the variable is correctly reported as STRING by SysVar.getAll.
• setstring.tcl mirrors setfloat.tcl/setbool.tcl (dom.GetObject(name).State(value)).

Registered in methods.conf alongside their enum counterparts, both at LEVEL=USER — consistent with the existing setBool/setFloat/setEnum privilege. They expose only typed string access (no arbitrary script execution), so there is no new privilege-escalation surface.

Changes

• WebUI/www/api/methods/sysvar/createstring.tcl (new)
• WebUI/www/api/methods/sysvar/setstring.tcl (new)
• WebUI/www/api/methods.conf (+2 method registrations)

Testing

Validated by structural review against the existing enum/float/bool methods and the WebUI's string-creation path. Note: I have not been able to run this against physical CCU hardware — a maintainer round-trip test (createString → getAll shows type: STRING → setString → getValue → deleteSysVarByName) as a USER-level session would be the ideal confirmation before merge.