Fix ISO Control Numbers Being Interpreted as Float

[DEMOCODE675]

Fixes #546

Summary

This PR fixes a recurring issue where ISO control identifiers such as "7.10" were incorrectly interpreted as numeric values and converted to floats (7.10 → 7.1).
Section identifiers are structural strings, not numeric values. Converting them to floats removes trailing zeros and collapses distinct identifiers:
"7.10" ≠ "7.1"
"8.10" ≠ "8.1"

This PR ensures section IDs are treated as strings across all architectural boundaries and adds regression tests to prevent this issue from reoccurring(locally).

What This PR Changes

1. Fix numericise_ignore Off-by-One
   Updated:
   range(1, wsh.col_count + 1)

2. Enforce String Type at All Boundaries

• In parse_export_format, explicitly cast:
  sectionID = str(sectionID)

In Standard.__post_init__
In Tool.__post_init__
Normalize:
self.sectionID = str(self.sectionID) if self.sectionID is not None else ""

3. Added Regression Tests
   New test file:
   section_id_string_test.py

Covers:

• YAML round-trip preservation
• Model invariants
• Parser string enforcement
• Natural sorting behavior
• numericise_ignore off-by-one fix

Test Results
All relevant test suites were run locally:
36/36 tests passing
The new tests explicitly cover the exact failure modes described in issue #546 .

Why This Approach

Rather than fixing only the spreadsheet.py behavior, this PR:

• Fixes the root cause
• Adds defensive programming at architectural boundaries
• Introduces regression tests
• Protects against silent reintroduction of float coercion
  This ensures long-term stability and correctness of ISO control identifiers.

Result

• "7.10" remains "7.10"
• "7.1" remains "7.1"
• No numeric coercion
• No identifier collapsing
• Regression tests prevent recurrence