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/mleaderstyles.py

@@ -11,6 +11,8 @@ from __future__ import annotations
 import math
 from typing import Any
 
+from engineering.standards.names import check_name
+
 __all__ = ["MLEADER_KEYS", "MLEADER_PRESETS", "resolve_mleaderstyle"]
 
 MLEADER_PRESETS: dict[str, dict[str, Any]] = {
@@ -20,8 +22,6 @@ MLEADER_PRESETS: dict[str, dict[str, Any]] = {
 
 MLEADER_KEYS: tuple[str, ...] = ("arrow_size", "landing_gap", "text_style", "text_height")
 
-_ILLEGAL_NAME_CHARS = frozenset('<>/\\":;?*|,=`')
-
 
 def _positive(key: str, value: Any, *, allow_zero: bool = False) -> float:
     if isinstance(value, bool) or not isinstance(value, (int, float)):
@@ -51,14 +51,7 @@ def resolve_mleaderstyle(preset: str, overrides: dict | None = None) -> dict[str
                 f"overrides: {raw_key!r} is not a leader style key; allowed: {list(MLEADER_KEYS)}"
             )
         if name == "text_style":
-            if not isinstance(value, str) or not value.strip():
-                raise ValueError("text_style: a text style name cannot be empty")
-            bad = sorted(set(value.strip()) & _ILLEGAL_NAME_CHARS)
-            if bad:
-                raise ValueError(
-                    f"text_style: {value!r} contains characters DXF forbids in a name: {bad}"
-                )
-            values[name] = value.strip()
+            values[name] = check_name(name, value, what="text style name")
         elif name == "landing_gap":
             values[name] = _positive(name, value, allow_zero=True)
         else: