fix(task-19): a .dxf template really reaches the live seat; the fallback dimvars land on Standard

Documents.Add(<file>) accepts only a genuine DWT. Measured on AutoCAD 2026:
Add(templates/iso_a3_pid.dxf) returned the default acadiso drawing (layers
['0'], Layout1/Layout2) exactly as Add() and Add(<missing.dwt>) did, so
drawing_new(template=<bundled name>) reported ok:true with source
'bundled_dxf' over a blank document, and the explicit .dxf path route was
just as blank. ComBackend.drawing_new now converts any non-.dwt template
through AutoCAD itself (Documents.Open read-only -> SaveAs ac2018_Template,
older constants tried next -> Close(False)), caches the .dwt in the temp
folder keyed on path/size/mtime, hands Add the .dwt, and refuses a missing
template file instead of letting Add default silently. The tool result
carries template.dwt / template.dwt_cached. Re-measured end to end through
server.drawing_new on the real ComBackend: ansi_b_mech -> 11 layers, tab B,
DIMTAD 0 / DIMTIH 1 / DIMEXO 1.5; iso_a3_pid -> 17 layers, tab A3, second
call reuses the cached .dwt (0.8 s conversion once). Fake-ActiveX tests pin
the call shape and that the conversion document never stays open.

Headlessly the build's fallback wrote spec 4.1 only as header DIM variables,
and the renderer folds six of them, so DIMTAD/TIH/TOH/EXE/EXO/GAP/LUNIT/LWD
were a no-op: ansi_b_mech dimensioned with ISO text placement while
system_get_variable and AutoCAD said ANSI. The fallback now also stores each
value on the Standard dimstyle; the five DXFs are rebuilt (--check passes)
and a test reads the effective values off a DIMENSION drawn from each
template and asserts header and style agree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

templates/ansi_d_arch.dxf

@@ -489,7 +489,7 @@ $SKPOLY
   9
 $TDCREATE
  40
-2461305.9168518516
+2461305.9345486113
   9
 $TDUCREATE
  40
@@ -497,7 +497,7 @@ $TDUCREATE
   9
 $TDUPDATE
  40
-2461305.9168518516
+2461305.9345486113
   9
 $TDUUPDATE
  40
@@ -929,11 +929,11 @@ $PSTYLEMODE
   9
 $FINGERPRINTGUID
   2
-{5DBCBFFD-85BB-4756-97CD-DF14393BA48D}
+{EA8DAB3B-221B-4973-A128-0F97D36F69C3}
   9
 $VERSIONGUID
   2
-{8BCC5654-8976-41B9-B27D-2DE6DFEDDA75}
+{41F085E9-E128-43F7-931E-FEE570A4F621}
   9
 $EXTNAMES
 290
@@ -2157,13 +2157,13 @@ Standard
  40
 1.0
  41
-2.5
+3.0
  42
-0.625
+1.5
  43
 3.75
  44
-1.25
+1.5
  45
 0.0
  46
@@ -2175,7 +2175,7 @@ Standard
  49
 2.5
 140
-2.5
+3.0
 141
 2.5
 142
@@ -2189,7 +2189,7 @@ Standard
 146
 1.0
 147
-0.625
+1.0
 148
 0.0
  69
@@ -2201,15 +2201,15 @@ Standard
  72
 0
  73
-0
+1
  74
-0
+1
  75
 0
  76
 0
  77
-1
+0
  78
 8
  79
@@ -3425,7 +3425,7 @@ DictionaryVariables
 280
 0
   1
-1.4.4 @ 2026-09-21T19:00:16.265081+00:00
+1.4.4 @ 2026-09-21T19:25:45.818106+00:00
   0
 DICTIONARYVAR
   5
@@ -3437,7 +3437,7 @@ DictionaryVariables
 280
 0
   1
-1.4.4 @ 2026-09-21T19:00:16.272301+00:00
+1.4.4 @ 2026-09-21T19:25:45.824109+00:00
   0
 ENDSEC
   0