Initialize TOC when file variables enable the extra

[agammann]

Problem

With use_file_vars=True, a document beginning with <!-- -*- markdown-extras: toc -*- --> crashes while rendering its first heading (UnboundLocalError: header_id). Declaring header-ids, toc in that file variable instead crashes because _toc_depth was never set. TOC setup currently runs only for constructor extras, before file variables are read.

Change

Run the existing TOC setup when extras are refreshed, so file variables get the same implied heading IDs and depth initialization as constructor extras. Add a regression covering both file-variable forms and repeated conversions with the same Markdown instance. Document the TOC file-variable form in the README.

Verification

• Reproduced both crashes on upstream d030ca7f9ef55b73f143450c5835e48f4b5d4043 before the fix.
• py -3.14 testall.py from test/: 289 passed.
• py test.py -- -knownfailure from test/: 289 passed.
• --use-file-vars CLI rendering emits a TOC and heading ID for the example modeline.
• git diff --check and changed-line PEP8 check passed.

[nicholasserra]

Thank you!