refactor(checks): split platforms_mirror/typescript_table.py and gate.py toward max_lines = 1 #1064

Step under #1020. Lane C, internals/checks/src/checks/platforms_mirror/ — PR 3 of 3.

Measured against origin/main at f575c06 with the ceiling set to 1
(npx -y testing-conventions@0.0.111):

typescript_table.py:31 `_without_comments` runs 4 lines
typescript_table.py:39 `_as_json` runs 2 lines
typescript_table.py:44 `typescript_table` runs 15 lines
gate.py:23             `run` runs 22 lines

Four flagged violations, four moves. typescript_table.py keeps
typescript_table and sheds _requoted, _without_comments (with the _TOKENS
pattern it owns) and _as_json; gate.py keeps run and moves read out.

Stacks on the decide.py slice: that PR rewrites gate.py's failure message,
which names the module holding SHARED by path, so both PRs touch gate.py.

No config change; no changelog fragment (internals/** is out of scope).