#2479 - Fix header/footer section parsing when &L is not first

[swmal]

See #2479

Problem

Header/footer sections were lost when not stored in Left, Center, Right order.

ExcelHeaderFooterText's constructor took the first section code from the first two
characters, then scanned for further codes — but only recognized &C and &R, never
&L. A &L appearing anywhere other than position 0 was not treated as a new section.
Its content was swallowed by the preceding section, then discarded when that section was
normalized, taking any &G picture placeholder with it. The picture survived in the VML
collection, so Pictures.Count was unchanged, but nothing referenced it and it stopped
rendering.

Excel writes sections in creation order, so ordinary Excel-authored files were affected.

Fix

Three changes in the constructor's parse loop:

• &L added as a section delimiter (the actual bug)
• pos = startPos - 1 — the loop's pos++ previously skipped a character, missing a code following an empty section
• bound text.Length - 1 — a code in the final two characters was never seen

Tests

HeaderFooterSectionOrderTests covers ordering permutations and both boundary cases