Fix text margin arrays to use documented TRBL order

[guiwoda]

Summary

• Text margin arrays are documented as TRBL [top, right, bottom, left], and table/slide-number margins already map that way, but textObjectToXml still used the old LRBT mapping (lIns ← margin[0], tIns ← margin[3]).
• That swap turns intended left padding into a top inset. Callers that convert CSS padding-left (e.g. hanging-indent bullet lists via dom-to-pptx) can then overflow a box whose height was measured without that inset.
• Issue TRBL margin for text object #551 claimed this was fixed in v3.0; docs were updated to TRBL, but the text-body implementation was not.

Test plan

Per TESTING.md:

• npm run ship
• Browser or Node demos → Text slides
• Confirm margin:[5,5,5,40] box shows a large left inset (not top)
• Confirm bullet example with margin:[0,0,0,10] has left padding only

[guiwoda]

Dropped the package.json test script (and incidental lockfile touch) — that was out of scope. The regression still lives at test/text-margin-trbl.test.mjs and can be run with:

node --test test/*.test.mjs

[guiwoda]

Updated testing to match TESTING.md / the PR template:

• Removed the standalone test/*.mjs unit test
• Added a TRBL text-margin case in demos/modules/demo_text.mjs (asymmetric margin:[5,5,5,40] — left inset should be large, not top)
• Annotated the existing bullet margin:[0,0,0,10] example

Verify with the Text demos after npm run ship (browser demo or demos/node npm run demo).