geometry: accept tab as a numeric field separator

[KJ7LNW]

Description

Decks that separate geometry-card fields with tabs failed to load. Any card whose
numeric fields are tab-delimited aborted parsing at the first tab, so otherwise
valid models could not be solved. Spaces and commas already worked; tab was
simply absent from the terminator set used to close a numeric field.

The integer and real field scanners each carried their own inline terminator
test, so the two could disagree. This change gives both one shared separator
rule and adds a permanent regression test through the public file parser.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Implementation Details

• src/c_geometry.cpp: adds the file-local query geometry_field_separator(),
  which classifies std::isspace() whitespace, the NEC comma, and the NUL
  terminating the loaded card. The character is converted to unsigned char
  before classification so negative plain-char values never reach the C
  character-classification interface. Both post-conversion scans in
  parse_geometry_card_line() call it, so the integer and real paths cannot
  drift apart. Numeric syntax checking stays in the existing scanner loops.
• src/nec2cpp_tb.cpp: adds the regression described below.

Reproduction

<TAB> denotes one horizontal-tab byte, 0x09. Original failing deck line from
86-2-ex9.nec:

SP 0<TAB>0 .13795 .13795<TAB>.98079<TAB>78.75<TAB>45.<TAB>.11957

Before: parsing aborts at character 5, the tab immediately following the first
integer field:

GEOMETRY DATA CARD "SP" ERROR: NON-NUMERICAL CHARACTER '<TAB>' IN INTEGER FIELD AT CHAR. 5

After: the deck reaches its end card and reports TOTAL RUN TIME:.

Testing

The permanent regression writes this complete input to a temporary file, parses
it with c_geometry::parse_geometry(nec_context *, FILE *), and asserts the
segment count. It exercises tab termination in both the integer and the real
scanner.

GW<TAB>1<TAB>3<TAB>0<TAB>0<TAB>0<TAB>0<TAB>0<TAB>1<TAB>0.001
GE<TAB>0

Before: parsing aborts at the first tab after an integer field.
After: parsing completes with n_segments == 3.

cmake --build build --target nec2++ nec2++_tests
ctest --test-dir build --output-on-failure

Result:

All tests passed (568 assertions in 52 test cases)

• Tests pass locally
• New tests added for new functionality