fix(task-9): finite text-style numbers, one symbol-name rule, canonical DIMBLK arrowheads

Three review findings on the standards data layer, each a value the layer
let through that a backend would then write as a silent wrong number or a
drawing that can no longer be saved:

* validate_textstyle never checked math.isfinite, unlike its siblings; nan
  and inf passed every comparison and reached the STYLE record verbatim.
  It now has the same _number helper dimstyles and mleaderstyles use.
* The three private _ILLEGAL_NAME_CHARS copies omitted \n \r \t that
  security._FORBIDDEN_SYMBOL_CHARS holds, so 'a\nb' was accepted as a text
  style, DIMTXSTY or leader text style and ezdxf refused the save. The rule
  now lives once in engineering/standards/names.py (fastmcp-free), the
  three modules import it, and a test pins its character set equal to
  security's and its output equal to illegal_symbol_name_chars.
* DIMBLK/DIMBLK1/DIMBLK2 were ("str",): any string went through, and the
  test pinned '_OBLIQUE', which ezdxf's writer refuses at doc.write while
  the COM engine wants exactly that spelling. They are now ("arrow",):
  canonical_arrowhead accepts a built-in with or without its underscore in
  any case (and '', '.', CLOSEDFILLED for the closed filled default), emits
  AutoCAD's documented block-name spelling (also what ezdxf reports after a
  load), and holds user block names to the symbol-name rule; ARROWHEAD_BLOCKS
  is pinned equal to ezdxf's ARROWS vocabulary, and ezdxf_arrowhead is the
  one translation the headless writer needs, proved by exporting all twenty
  headlessly and reading each back as its canonical name.

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

engineering/standards/__init__.py

@@ -10,12 +10,15 @@ the tools and the provenance tests read one copy.
 from __future__ import annotations
 
 from engineering.standards.dimstyles import (
+    ARROWHEAD_BLOCKS,
     DIM_VARIABLE_RANGES,
     DIM_VARIABLE_WHITELIST,
     PRESET_VARIABLES,
     PRESETS,
+    canonical_arrowhead,
     check_dim_value,
     describe_preset,
+    ezdxf_arrowhead,
     resolve_dimstyle,
     validate_overrides,
 )
@@ -23,14 +26,17 @@ from engineering.standards.mleaderstyles import MLEADER_PRESETS, resolve_mleader
 from engineering.standards.textstyles import TEXT_PRESETS, resolve_font, validate_textstyle
 
 __all__ = [
+    "ARROWHEAD_BLOCKS",
     "DIM_VARIABLE_RANGES",
     "DIM_VARIABLE_WHITELIST",
     "MLEADER_PRESETS",
     "PRESETS",
     "PRESET_VARIABLES",
     "TEXT_PRESETS",
+    "canonical_arrowhead",
     "check_dim_value",
     "describe_preset",
+    "ezdxf_arrowhead",
     "resolve_dimstyle",
     "resolve_font",
     "resolve_mleaderstyle",